So, the comics have had, over the years, some pretty amazing monsters of their own. Creatures that would usually be viable opponents for high-level RPG play, fantasy or otherwise.

Is there any so memorable that you'd want to use it, or maybe HAVE used it, in an RPG campaign?

For me, its the Sun-Eater. I'd use that sucker (and the whole idea of it being created by the Controllers, super-powerful extra-planar beings, often gone rogue or mad) in a high-level D&D campaign.
Of course, I HAVE used it in my Legion of Super Heroes campaign, but that's kind of cheating, seeing as how that's where it came from.

So for the purposes of this thread, we're talking about using comic-book monsters in any type of game OTHER than supers.

RPGPundit

Yes. Venom.

He would deserve a character/monster type/template based on some form of infection/disease of its own. That trip is so cool it has to make it in D&D somehow.

I can see Starro fitting nicely into a D&D style fantasy setting.  A whole village, town, or city under Starro's control would make for an interesting adventure, especially for *Good alignment PCs, who would have to avoid trying to hurt the innocents who /can/ be freed later.  

The Phalanx, or Technarx, whatever they call them these days, could be either interesting or utterly horrifying for a sci-fi setting.  I don't play Transhuman Space, but it might be a good fit.

Anything (more or less) from Neil Gaiman's various Sandman/Dreaming/Books of Magic works would be great for either Unknown Armies/nChangeling style Urban Fantasy, or a more traditional Fantasy setting.

The Brood.

http://www.mutanthigh.com/aliens/brood.html

Regards,
David R

hellcows from howard the duck!

Dire Wraiths: A shapeshifting race that can stick an acid tongue in your skull and absorb your memories? The men are masters of technology so in a fantasy setting they may have the best arms and armor, even things humans don't yet have--like early era firearms. The females are masters of magic, add to that they have a dragon like form (Shadowy Pterosaurus that breaths flames a Deathwing), and can transform dogs into hunters that can phase through objects.  

Very powerful and if used right a serious danger. I've used them before, and found them very effective for low key games, and making people paranoid. Especially when the heroes find clothes that resemble their allies or friends covered in the dust/ash of a Dire Wraith victim.

For my next campaign, I've given my goblins a twist into the direction of the Goblin/Green Goblin/Hobgoblin/whatever that Osborne dude or his offspring or his secretary or whoever owns that damn glider, mask and the megaton of bombs is called these days: Nasty little fellas with a wicked sense of humor and an even worse sense of fashion than your average superdude.

Warbands will of course also have access to flight machines, and the standard weapon is the thrown bomb, with goblin shamans being skilled at the creation of various funny kinds of explosives and poison gasses. (I might be a pervert for that, but I generally love to include gun powder into fantasy campaigns. It's not just historically appropriate, with it's smoke development it's also a cool kind of explosive, and it also allows for various funny side uses apart from straight blowing up and musket actuation.)

The Mole Men from Fantastic 4 could be another good one.

RPGPundit

I have done it in reverse, fantasy game back to supers.

Gareth from Labrynth appeared in my Supers Games and if it wasn't for a fluke would of stomped them to the ground.

The Fey courts showed up in my supers games.

Ice Giants.  It was not fun.

An honest to God, 22HD dragon gave my players pause until they realized they were super heroes. Then after a few minutes they realized it was just that powerful and became scared.

Any number of shadowbeasts and flowed back to my world.

Kaiju from the Old Marvel Days have shown up in my Agency Games (Super Agents).  

Going the other way?  Well one of the Big Bads in my fantasy campaigns is a super villian, a transdimensional, magic wielding, technoarmor wearing, supervillian.  He slows my supers characetrs down and crushes the fantasy guys unless they are very, very careful and avoid direct confrontation without a distraction.