Lampshaded during the Onslaught saga; Joseph was telling Captain America about being rescued by someone and Cap interrupted to say he knew it was Dr. Doom. When Joseph asked how Cap knew that, Cap's answer was "Frankly, the entrance line alone told me that much."
Also lampshaded during a cameo appearance in The Amazing Spider-Man, in which Doom expresses displeasure at being escorted through the public terminal of an airport due to mechanical difficulties (amusingly, the guard turns out to be Captain America undercover.):
Doom: Typical shoddy American workmanship. Such incompetence would not be permitted in the Latveria of — DOCTOR DOOM.
Guard: How do you do that?
Doom: Do what?
Guard: Speak in all capitals like that?
Doom: Silence, minion.
In an essay about his run on Fantastic Four, Mark Waid once noted that "you know you've Doom's voice down when every single sentence contains at least one pompous adjective."
Norman Osborn has shown surprising talent for this trope when he staged an attack by the Green Goblin on Air Force One. It's filled with such delightfully hammy lines as "GET BEHIND ME MR. PRESIDENT!" and "No Goblin! What YOU need is YOUR GLIDER!" However, when it's Osborn in the Goblin suit, the ham quotient goes up to 11. As does the insanity quotient.
Super-strong characters tend to get this way, especially those who are gods. Marvel has Hercules and Thor, DC has Orion of the New Gods, and now the Dark Avengers have Ares. Who possibly tops Thor for sheer hamminess.
Thor has the excuse that he comes from a worldof ham — Asgard. Where you know the inevitable fate of the scenery the moment anyone at all opens their mouth.
Of all the characters that qualify for this trope, Darkseid deserves a special mention.
Falstaff (again) in the Seven Soldiers of Victory story from Leading Comics #14, "The Bandits from the Books." In a story where everyone talks in all capitals and ends their sentences with exclamation points, his dialogue is in bigger, bolder type with more exclamation points. He even makes eating a banana sound epic.
The X-Men villain Mojo. Even in the Ultimate Universe where he's human. If the Joker were a fat, alien media tycoon, this is who you'd get. Mojo runs a world where people are engineered to be reality TV stars. Needless to say some of these people are quite the large hams when they come over to 616, Shatterstar for example
Rasputin in Hellboy has a tendency to ham it up even when he's losing. In his final appearance, as a ghost, he was shouting a hysterical tirade at a god. She didn't take it too well.
Loki is displeased that thou hast not included me into this page.
Spidey gets to ham it up and have fun doing so in his Marvel Adventures team up with Doctor Strange. In fact, the level of ham becomes essential in bluffing a Cosmic Entity, who is blind and can't see how small the "Great Hidden One Known As The Spider Man" is.
For that matter, Strange himself, on many occasions.
The Blue Blade from The Twelve or, as he would announce, "The Bluuuue Blaaaaaade!!!", was an Errol Flynn wannabe, with the camp elements of his original design turned up to eleven for laughs.
Sergei Korolev is like this in public in the graphic novel Laika ("All because of that lying b*stard, GLUSHKO!"), but in private, it turns out he's actually much quieter and very sad.
I swear to God! I spent serious quality time thinking about ways to TORTURE your irritating, BOMB-THROWING BUTT TO DEATH!
Brother... I am ready to TORCH this hell hole into ASH... to KILL every sad mouth breather who was dumb enough to live here... and I'm ready to GO DOWN WITH THE SHIP. And between the two of us, little man... we both know I'M the one who's not afraid to die.
Haazen from Knights of the Old Republic is one; during that sequence when he reveals his evil plan and blows up the courtyard with the fleet, he screams, "Let the Fire of Truth Rain Down." Of course, the man's spent almost his entire life forced into the role of the fawning servant, unable to act otherwise in public without blowing his cover. Under those conditions, the culmination of the Evil Plan he's been working on for decades deserves a bit of ham.
Justice League International villain Manga Khan is not only the founder and former president emeritus of the Manga Khan School of Melodrama, he suffers from a disorder that causes him to behave in a ridiculously grandiose manner. He neglects to take his medication because he's in denial.
Les Légendaires have several of these. One of the most notable examples includes Captain Shamira, Shimy's mother, who often delivers Badass Boasts in an overly aggressive tone.
Of course, considering Evil Is Hammy and what the villains are, Darkhell, Skroa, Anathos and Abyss all displayed large ham tendencies. Notably, most of them are usually quite calm (though still having a taste for Evil Gloating), but will easily go full large ham when excited or angry.
Elysio had a very hammy moment at his introduction (which was most likely a foreshadowing of the fact he was actually an amnesiac Darkhell). After Darkhell and him split up, however, he seems to have lost this tendency and become calmer.
The titular character in The Tomb of Dracula was incredibly hammy even by most vampire standards. Every issue had him speaking in terms like these:
Dracula: Feel HONORED clod—Be PROUD that your pitiful life can be used to further MINE. You did not DIE for any peasant...but for Dracula—LORD OF THE DAMNED!
Pinkie Pie turns up the ham when she confronts her nightmares in Issue #6.
Pinkie: I am the MOST laughter-giving, fun-having, friend-making pony ever! YOU HAVE NO POWER OVER ME!
"ANNIHILUS IS THE WILL AND THE WAY AND HE WILL NOT BE DENIED!" Marvel's Annihilus always refers to himself in the 3rd person, always shrieks at the top of whatever it is giant purple-and-green bugs have instead of lungs, and got a whole Cosmic Crisis Crossover named after him, during which he murdered billions and threw frequent temper tantrums because all of creation wasn't dying quickly enough.