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Cell: You fool! Have you forgotten? I know you! Your cells are woven into my very being! You, Vegeta, are a part of me! Vegeta: Now I'm going to take you apart! How ironic!
"Maybe if this were five-hundred times Earth's gravity, you would have the advantage. But ten? I don't even feel it."
A few minutes before the above: "Welcome to the end of your life. And I promise, it's going to hurt."
Gohan also delivers an immensely badass one to Super Buu when he arrives on the battlefield after Old Kai unlocks his potential. Buu says to him "So hotshot, you want to fight Majin Buu?" and Gohan responds with "Fight you? No, I want to kill you," and then lays the biggest beating on Buu that he had probably ever experienced.
The Prince of Tennis: Ryoma's "Mada mada dane" ("You still have lots more to work on") before turning around and wiping the court with an opponent he was losing against up until that moment.
Another Ryoma, though from Getter Robo. He purposefully grabs hold of a yakuza boss' knife blade and tells him "What's the matter? Come on, either push or pull. You'll save face if you at least chop off one finger" before kicking his terrified ass and scaring off the rest of his thugs.
You really don't want to be around when Sesshoumaru of InuYasha fame says, "It seems I've been underestimated".
HaoAsakura of Shaman King has a minor example with his, "So tiny/small" line. Not sure if this counts since it's not restricted to being said pre-battle; indeed, it's been said as a response to an attack mid-battle, or as a post-battleOne-Liner. But it seemed appropriate.
Touma: I will break it... That messed up illusion of yours!
Accelerator has two while curb stompingAwaki: "But, you know, just because I've gotten weaker doesn't mean you're any stronger now does it?" and "Sorry, but this is a one-way-road!" (NOTE: The Alternate Character Reading for the kanji used to write Accelerator's name reads "ippotsuko" or "one-way-road").
Fushigi Yuugi: "WHAT—DID YOU DO—TO—MIAKA? I asked you a question, DAMN IT!"
Several in Bleach. For example, Aizen before rendering Hitsugaya a bloody mess. "Don't use such strong words. They make you look weak."
Ichigo after Orihime cries out for him not to die. He grabs the blade and... "Sorry, Grimmjow. Looks like I can't let you hurt me anymore." *SLASH*
Rangiku after Izuru has rendered her sword unusable. "So what if I can't hold my weapon to use it? I just won't hold it." She then proceeds to unleash her shikai and own Izuru with it.
Way back when Ichigo and Sado first became friends. "Hello, I need an ambulance by the bridge. How many? *Counts the thugs holding Sado hostage* Five of them."
In Tengen Toppa Gurren Lagann, every hero in the show, at one point or another, gets to yell some variation of "WHO THE HELL DO YOU THINK I AM!!". This culminates in the speech before the final fight, where every Dai-Gurren member gets one line in their epic Badass Boast speech.
The scene in Yu-Gi-Oh! where a villain is talking about how he can use his powers to seduce any woman in the world — after knocking out and planning to rape Tea/Anzu. Yami is waiting by the door, listening to the whole thing: "You think so?" Asskicking ensues.
The Toei anime gave Yami Yugi the catchphrase "Saa, GAME no jikan da" (So, it's game time) which usually meant the dice were fixed, the cards were palmed, and you were probably going to wish you could die. The manga translates this to 'Let's play a game' instead. The Manga also has a pair of one liners for him, bookends style. He would open with 'Game Start' and end it with his Pre Asskicking Pose and the Pre-Mortem -I Mean, insanity one liner, Penalty Game.
"The first thing I'm going to burn is your tongue." In Fullmetal Alchemist, Mustang says this to Envy when he finds out that the latter killed Hughes.
Mustang takes this to a higher plane. He pulls one off with the simple words "Too slow," before proceeding to nuke Ed, the stadium, and a whole lot of spectators in a barrage of fiery explosions.
And the end of the manga with the fight against Father. All of the alchemists are pounding on Father like there's no tomorrow, Ed's automail arm is completely destroyed, Al sacrifices his soul to bring Ed's real arm back, and at this point, Edward is majorly pissed, and brings back a line from the very first chapter:
GET SET. "I'm sorry, and thank you, Erio, Caro. There's nothing to doubt, right?" RIOT ZANBER.*Cha-CHACK.* "Because I was weak, I was hesitating and worrying. I will probably always repeat this. But. That's fine... Because all of that is me."
The perhaps most epic one: In Enies Lobby, Luffy is informed that its garrison numbers in the tens of thousands.
Luffy: And I'm alone. Get out of my way!
There's also post-timeskip Luffy who gives us one with only one word. He's fighting the same exact pacifista that defeated and scattered all nine Straw Hats two years before. It shoots a laser made of light at Luffy who remarks; "slow." Then lays it down for good with one jet pistol.
Admiral Kizaru delivers an awesome one before kicking the crap out of Basil Hawkins:
Speed is... "weight". Have you ever been kicked at the speed of light?
Luffy stands atop a building, calling out for Bellamy who asks Luffy if he "even knows how to throw a punch", causing the observing crowd to burst in laughter. Luffy simply says "Do I know how to throw a punch?" and proceeds to one-hit Bellamy, the crowd instantly goes silent.
Zoro also has many of these. For example this golden line in the Fishman Island battles, when he fights an eight-sword-wielding fishman. Zoro slices the fishman's swords horizontally and stares very scarily at him.
Given how much asskicking there is in Maiden Rose these show up fairly often. One of the more memorable ones is after Berkut recovers from the train being derailed by two anti-tank missiles and comments that Taki must be insane to have done that. Cue Taki standing on top of the wreckage with his katana adding, "and yet, I still haven't managed to finish you off, have I?" before a Curb-Stomp Battle ensues.