Light Yagami of Death Note later falls into this after becoming a detective.
Campanella Freuling (female) from DOGS: Bullets & Carnage.
Inspector Verossa Acous from Nanoha StrikerS is a heroic example.
The Hückebein family from Nanoha Force are a villainous one, however. Apparently, they are good enough at murder that slaughtering an entire facility of mooks in chapter 17 didn't leave a single stain on their badass suits.
They aren't very badass though, being trained for espionage, not combat.
Ayame and Kiri-chan from Ga-Rei -Zero-, both are female.
Most of the Magnificent 10 and a few of the good guys from "Giant Robo" take part in this trope. Alberto the Shockwave is probably the most prominent example.
Fullmetal Alchemist: Whenever Roy Mustang is not dressed in military uniform, he wears damn fine period suits. He even wears a dress shirt under his uniform while seemingly everyone else uses more casual t-shirts or sweaters.
On the evil side, once he is released from prison in the manga and new anime, Zolf Kimblee is such a smooth criminal that he always dresses in a snazzy white zoot suit and fedora.
Pride aka Selim Bradley, who is always (when in human form, that is) dressed in a button-up shirt, a vest and dress shoes, and whom we even see in a suit on one occasion in Father's lair.
When he isn't dressing up as his sister, Johan from Monster is usually seen in a well-tailored suit (albeit sans necktie).
Valkyrie Lind of Ah! My Goddess. She's a professional badass, and she looks the part.
Soul Eater most obviously has Death the Kid, but Maka's uniform might also count, not to mention the positively Mafia-like black pinstripe suit Soul wears in his Black Bug Room with the grand piano.
Sebastian from Kuroshitsuji. Almost always dressed in a snappy tail coat, and able to end someones existence in several different ways.
From Noblesse, Raizel and those who side with him always dress in nice suits in combat.
Comic Books
Marvel's Kingpin always dresses in a white suit.
Batman's foe The Penguin and his trademark tux and top hat.
In the Claremont-era X-Men comics, Sebastian Shaw, the Black King of the Hellfire Club, always wore the height of Eighteenth-century fashion, complete with ruffled collars and sleeves. Most Hellfire high-rankers wore this style while meeting at the club (the men did— the women dressed less formally) but Shaw kept up the look all the time.
Whenever The Minutemen from One Hundred Bullets take a job, expect to see all of them wearing black suits and ties.
Many of Tarantino's characters: most of the cast of Reservoir Dogs (at least on the day of the heist), Jules and Vincent in Pulp Fiction, the Crazy 88s in Kill Bill...
Jason Statham's Frank Martin in The Transporter. In the second film he even has a spare one in his car to deal with the inevitable (non-fanservice) Clothing Damage.
Hell, the entire visual style of the film is built around this. It actually works as a plot point, too, juxtaposing the control of the main characters in the Matrix (both by machines and of themselves) with the desolation of the real world (where everyone is basically in rags).
Nicolas Cage's character in Snake Eyes changes from his sharkskin jacket to a suit jacket as a symbol that he's stopped messing about and is serious.
Collateral: Assassin Vincent wears a steel-gray suit and tie, but loses the tie as the situation grows out of his control. Michael Mann explains in the DVD Commentary that his look is designed to be as unremarkable as possible. So he's not just a Badass in a Nice Suit, but dressed to get away with murder.
Subverted in The Blues Brothers, where the brothers wear black suits, but it's soon revealed that those are the only clothes they own. They wear them to bed, into the sauna, etc. Unsurprisingly, they smell bad. Lampshaded in the sequel when Elwood tells his new partners why they have to dress the same way.
James Bond: James, much of the time at least. He even has the famous "fix my tie" maneuver.
Jef Costello, the hitman from Jean-Pierre Melville's crime picture Le Samourai never leaves his apartment unless he's dressed in a suit, tie, rain coat, and perfectly positioned fedora. For that extra bit of class, he always puts on white gloves before a kill. Jef cares as much about his appearance as he does his alibi.
In Die Hard, Hans Gruber points out that he's wearing an expensive suit from the same tailor as the one worn by the Japanese business executive.
Henri Ducard aka Ra's al Ghul in Batman Begins: hand-to-hand combat in a suit and tie.
Justified by the titular The Tuxedo, which is in fact the ultimate spy gadget, providing its wearer with every possible skill you'd need for a spy mission, from gun assembly, to kung fu, to wicked dance moves.
Storm Shadow from GI Joe The Rise Of Cobra dons a spiffy white suit sans tie in the Paris segment, while James McCullen though not being a fighter does have a nice dark suit.
The entire Inception team gets into this at various stages, including Ariadne. Some particularly notable ones are Cobb, Arthur, and Saito in black tie at their first meeting, the hotel dream levels, and Arthur's famous hallway fight, in which he wears a three piece suit and removes only the jacket.
Literature
Assassins in Discworld. If they don't wear black, they reason, they might as well just be thugs who kill people for money.
Subverted in Neverwhere. Croup and Vandemar are contract killers who wear suits, but the suits look like they were made by someone who had only had a suit described to them and so looked creepily off.
Not helping matters was that the suits were on Croup and Vandemar.
Kit Carson of Time Scout habitually wears a rumpled suit when working at his hotel. Anyone who goes downtime has a better than even chance of wearing a very nice suit.
"Gentleman" Johnny Marcone fits the trope to a T. The man's badass credentials are too extensive to list, but trust me, they're solid. Hand-tailored suits and tuxedos are his standard wardrobe.
Live Action TV
This trope was extremely common in American crimedramas of the 60s and 70s, particularly in representing characters from organized crime, who would never go anywhere, including an outdoor worksite, without putting on a full suit and tie. This trope could easily be called Wise Guys in Ties.
Lampshaded (unintentionally) in "The Frame," a very early episode of Mission: Impossible, where the team infiltrates a mob meeting at the boss's home and a major plot point hinges on one character getting spilled on and being sent upstairs to change into an entirely new suit, which the boss subsequently compliments him on.
Marcus Hamilton in the last season of Angel, who was more or less the Terminator with etiquette. After being told his friends were running from "a tall, well-dressed..." and then Hamilton bursting upon the scene, Angel remarked, "Wow. He really is well-dressed." The season previous, Gunn attended a fancy party, and in order to blend had to wear a suit...and then had to beat up a bunch of warrior monk-types, which he lamented would get blood on it. (It didn't, though. He's that good.)
The Hands of Blue from Firefly, though they only appeared in two episodes and a comic.
On Doctor Who, the Master sometimes wore a black suit and tie (instead of his customary black Nehru suit), especially when he started the story posing as a human.
Brother Mouzone in The Wire is possibly the epitome of this trope. Not only does he wear an immaculate three-piece suit at all times (with a very sensibly-sized handgun underneath), but also Nerd Glasses and a ''little red bowtie''. But do not underestimate his ability to fuck you up.
Dollhouse: In classic MIB style, all the handlers are, in Topher's words, "security guards in very lovely suits." Boyd and Dominic look especially good when Kicking Ass and Taking Names.
In the Battlestar Galactica remake, the Doral/"#5" Cylons are always impeccably dressed as is Gaius Baltar most of the time.
He's does tend toward polos when not "on the clock". It's a good look for him.
In one particular episode Michael, Sam and Fiona dress in all black Armani to give similar impression to that uniforms give in the battlefield. That is, the impression that one belongs to a larger organization and of unified force. They succeed, of course. Also, they look fantastic.
"White Collar" The whole show is this - considering that Peter and Neal are very rarely not in suits when in the thick of action.
Put simply, villains from Heroes seem to love this trope. We have:
Mr. Linderman, from Season 1
Adam Monroe from Season 2 (who really pulls it off)
Arthur Petrelli from Season 3 (who doesn't). Maury Parkman, too.
Noah Bennet wore nothing but suits for Season 1, and a large chunk of Season 3.
Suits were the default clothes for Nathan in Season 1, and most of Season 3.
Sylar also attempts this in volume 3, and does about as well as Arthur.
Finally, though this may be stretching it, Danko from Volume 4 wore suit pants with his leather jacket.
Some of the angels on Supernatural (Castiel, Uriel, and Zachariah) are always shown wearing suits. Castiel scores extra points with his Badass Longcoat.
The Thick of It: Malcolm Tucker looks remarkably un-scary—even emasculated—in casual clothes. Put the man in a suit, however, and he'll chew you the fuck out.
The Gentlemen from Buffy the Vampire Slayer float around ripping people's hearts out while dressed in sharp black suits.
Though they're all FBI agents (and all Badass), Aaron Hotchner of Criminal Minds is the only one on the team to consistently wear a suit throughout the show. He only takes off the jacket when he switches it out for a Kevlar vest. And in a handful of very early episodes, Derek Morgan played this trope before opting for more casual dress.
Zig zagged in TheSopranos ,the mobsters like to wear vulgar sportswear most of the time but they dress up according to their fancy businessmen status when the occasion requires it.
Killer7: Garcian Smith, Mask De Smith (along with a cape and luchadore mask), and Dan Smith.
No More Heroes: Another Suda 51 character, Henry Cooldown, whether it be a Badass Longcoat or a black and gray suit.
The Gman from Half-Life always wears a suit (and carries a brief case, just to add to the look), helping him stand out against the the generic scientists and security guards of the first game and then the ragtag resistance members of the second. He also remains decidedly professional (whatever his "profession" is) and calm compared to all the other characters.
Any Castlevania game where Ayami Kojima did the designs will have its lead character dressed in the very height of (somewhat) period-appropriate fashion, although special mention must be made of Alucard, both his dandyish dress from Symphony of the Night and his elegant double-breasted suit, complete with red pocket square, from the Sorrow games.
Mr. Burke from Fallout 3. While nearly everyone else in Megaton is a dirty, ragged-looking peasant, Burke is rather snazzy in his white suit, fedora, and tortoise-shell glasses.
"I just had this suit TAILORED!"
The Crimson Viper from Street Fighter 4 wears a business suit with lightning generators in her sleeves and rocket boots disguised as high heels.
Tekken: Brothers Kazuya Mishima and Lee Chaolan tend to wear very lavish suits while fighting; Kazuya wears a dark violet suit with long, angular tails, and Lee wears a black tuxedo with a rose on the lapel and silk gloves. It makes some sense, given their father Heihachi's attire (fur-lined tiger greatcoat with white vest-pants combo). Others who dress in this manner in later games include Baek Doo San (three piece suit and hat), Feng Wei (mafia-style suit and open-collared shirt), and Anna Williams (cocktail dress).
Almost all the mobsters in The Godfather: The Game wear suits with ties and hats. The Cuneos and Barzinis stay in this trope despite using red and green respectively because they stand by dark shades. Your character initially doesn't have a suit, but you can buy several kinds of coats.
In Scarface: The World is Yours Tony's Enforcer wears a grey suit and tie with shades, while Tony can purchase various tieless suits.
The first Gym Leaders in Pokemon Black and White are Cilan, Chili, and Cress. Since the gym itself is based on a cafe all of the trainers, including the leaders themselves, are dressed like waiters. And they don't look half-bad, either.
There's also Steven, Darach, Lucian, Riley and Giovanni (the latter two having fedoras), to the point they are difficult to imagine in any other style. Not to mention the waiters/waitresses and maids...
Persona 3 features the option to dress all the male characters in tuxedos.
Thane Krios wears what appears to be a space tuxedo. Space leather tuxedo, that is. His design process was long and hard to make him not only have the flair and confidence of an assassin, but also look intriguing to women. He even poses in his own personal idle animations.
Commander Shepard as well in his Cerberus officer uniform or the formal wear he procures from Kasumi's mission.
MordecaiHeller of Lackadaisy fame combines elements of this trope along with Sharp Dressed Cat, Career Killer, and Wicked Cultured. Utterly ruthless, he has absolutely no qualms about hacking a screaming victim to pieces with a hatchet. Priding himself on cleanliness and order, his only problem with the killing was the mess.
Each protagonist makes himself new clothes after acquiring enough alchemy ingredients. John gets a rather dashing teal suit, but Dave later one-ups him by making a tuxedo, complete with bowtie!
Heck, Dave has at least three outfits that could qualify for this trope, including the tuxedo.
The Midnight Crew, who live this trope with a healthy dash of noir and more than a touch of sociopathy (although in the case of Clubs Deuce, the "badass" part is ... debatable).
Same thing goes for the Felt, especially with Doc Scratch.
Web Original
Virgil, Bree's Watcher and sometimes Lucy, all from lonelygirl15.
The Shadow and the Watcher from KateModern, and Tez On Toast towards the end of The Last Work.
Anonymous is often depicted as a hollow tuxedo or black business suit.
Slender Man; at least one goon suspected he/it wasn't actually wearing a suit but had a very strangely-proportioned (and colored) body (for instance, his neck actually drooped like a vulture's, giving the appearance of a tie). This is just about the only element of him that appears in nearly all the stories.
Legend from the flash series Xin. In the second half he trades the full suit coat for a waistcoat, and when it's time to get serious he tears it off and rolls up his sleeves.
The Perils of Penelope Pitstop had the Ant Hill Mob, always formally dressed and carrying Tommy Guns like the good little stereotypical mobsters they were.
Parodied on one episode of The Venture Bros.: "I love killin' guys in a tux, makes me feel like James Bond."
Justice League Unlimited: Lex Luthor: "I had to go get my Power Suit."
Charles Foster Ofdensen of Metalocalypse is very much this trope.
CIA Agent Stan Smith on American Dad always wears a suit.
Bishop in Teenage Mutant Ninja Turtles 2003. He takes on all of the turtles, Splinter, Leatherhead, April, and Casey with nothing but a suit and a tie. Shredder needs his armor, foot army, and Karai/Hun to take on the Turtles alone. Truly a badass.
Brick in Young Justice. He is, needless to say, quite upset when the suit is ruined in battle. Do you know how much it costs to get a suit in his size?
Real Life
There's a famous photo of strikers battling scabs and strikebreakers at one of the Detroit auto factories during the Depression. Everyone in the riot/battle is wearing a suit and tie.
