I enjoyed the American Eagle vs. Bullseye fight in Thunderbolts.

Dardevil vs that demonic subway train/dragon in the Nocenti/Romita Jr. run of DD. It was a crossover with one of those mutant events where hell spilled over into New York City.

Daredevil vs Ultron, during the same run, for that matter. It was during the Acts of Vengeance crossover.

The Society pulverizing the Uncle Sam & the Freedom Fighters in Infinite Crisis #1

Edit: OMG I didn't know this was a Marvel only fights.
In that case any Ultimates fights

Best fight in comic history...

Thanos Vs The Remaining Heroes durning the Infinity Guantlet. The godly pimp-smack that Thanos lays on Cap at the end of the fight is priceless.

Runner-up...

Wolverine Vs Sabretooth durning the Mutant Masscure. Most brutal fight ever.

Oh man. That Bullseye vs. American Eagle fight was great! I thought bullseye was gonna own that guy (as he just took out Jack Flag) and it was hyped that he would. I did not know that Bullseye wasn't even in the same league! American eagle just took him down with ease....even being strong enough to break that back. Great fight indeed.

Spidey vs. Firelord was over a pizza right? I think I heard of that one.

I would love to read mutant massacre. I heard about the Sabes/Wolvie fight, but have yet to witness it.

Other goodies:

Songbird vs. Green Goblin- I thought she'd be outta her league, but I was wrong. I like her the more I see her.

Swordsman vs. anybody- The newest swordsman talked so much noise only to get beaten....every....single...time. I was really waiting to see who he would beat, but he never shined up to his "death". He had heart though. I'll give him that.

I liked Thanos vs. everyone in the Infinity Gauntlet as well. A very unique death to cyclops there and a lot of others "getting offed" while Surfer watches.

Wolverine vs the X-Men, "Wolverine: Enemy of the State"

Wolverine and a re-programmed Sentinel vs The Hand, "Wolverine Agent of SHIELD"

The Ultimates vs The Hulk, "Ultimates 1"

Ultimate Captain America vs Ultimate Giant Man, "Ultimates 1"

The Ultimates vs The Liberators, "Ultimates 2"

Nova Vs Annihilus, "Annihilation #6"

New Avengers vs The Hood's Army, "New Avengers" #50

AoA Magneto vs AoA Apocalypse, "X-Men: Omega"

Bishop and Assorted X-Men vs Magneto, "Uncanny X-Men" #304

Havok vs Random, "X-Factor(1st Series)" #88

And those are the ones that come to mind right off the bat. Possibly more later...

One from one of the first comics I ever owned The New Defenders # 134:

The New Defenders Vs. Manslaughter.

Ooh! Blink vs. Holocaust in Astonishing X-men #4 (AOA). An awesome fight showcasing how teleporters should be represented.

New Mutants #37 - New Mutants versus the Beyonder in one of the more entertaining Secret Wars II tie-ins. Really, it's not much of a fight. The Beyonder wipes them out pretty casually. But I still remember the panel in which Dani Moonstar (who I think is the last one still standing) charges him on her winged horse.

Uncanny X-Men #208 & #209 - X-Men versus Hellfire Club versus Nimrod in Central Park. Two X-Men get knocked out of action and Nimrod gets hit by a meteor named Sebastian Shaw. And these two issues segue directly into the Mutant Massacre.

Is mutant massacre in a trade? Sounds like some really good stuff in that comic arc.

Blink's fights are always good. Towards the end of her second stay on the team she got all "nightcrawler" on people and started rushing them from all sides (I'd never see her do this in the beginning exiles books). I would really like to see her fighting style in this new version.

oh, son! Is Mutant Massacre great? I say thee, YAY!!!

All the fights in Uncanny 210-211 are great and form the top part of my list.

I liked Book of Ezekiel where Spidey fought Morlun. That was good stuff.

Wolverine v. Gorgon in Enemy of the State was sweet.

Wolverine v. Lady Deathstroke in Uncanny 206.

Ultimates vol 1 fight against Hulk.

Ultimates vol. 3- Ultimates take down Thor.

I actually liked the World War Hulk fight between Hulk and FF, and Hulk v. Stark. Brutal and JRJR drew it with a lot of energy.

I was totally going to mention this.

The X-men square off against the Hellfire club in Central Park, and the sheer number of mutants in a small vicinity sends Nimrod to investigate.

Nimrod pops up and immediately fries one of the Hellfire club's bishops (or maybe rook?) then IIRC Nimrod throws Black King into orbit. Another Hellfire club member (Harry Leland?) dies from a heart attack from the sheer insanity of the fight but manages to bring the King back down from Orbit. While all this is going on PhoenixII gets kidnapped by Spiral... yeah.

The X-men got their butts handed to them and never fully recovered. They went into the mutant Massacre storyline a broken team, and just got more torn up.

Good times.

That was also where Nightcrawler got himself really hurt by trying to teleport a piece of Nimrod away (can't pull that twice, Kurt). And Thus Excalibur was born. Ah, good times.

New Mutants Annual 3

Impossible Man vs. Warlock

'nuff said.

Bendis/Maleev doing DD vs. Bullseye, when we find out his name is Lester. That was a beatdown, so maybe not quite a fight... but really enjoyable.

Ms Marvel vs. the giant superskrull-thingies during Secret Invasion.