Avengers vrs Korvac - Avengers 176
Wonder Man vrs Vision - Avengers 158
Batman vrs Joker - The Dark Knight Returns
Spiderman vrs White Tiger - Spectacular Spiderman 10
Thing vrs The Champion - Marvel Two and One Annual 7
Shang-Chi vrs The Cat - Master of Kung Fu 39
Captain America (Johnny Walker) vrs The Watchdogs - Cap 345
Iron Fist vrs Steel Serpent - Marvel Team-Up 63-64
X-Men vrs The Brood - Uncanny X-Men 166
The Thing vrs The Thing - Marvel Two and One 50
(Not the most epic battles, but some of my personal favorites)
Juggernaut vs Colossus (bar room brawl)
Sabertooth vs Wolverine
Superman vs Doomsday (Death of Superman)
Morg vs Heralds of Galactus (death of Nova)
Superman vs Captain Marvel (Superman Eclipsed)
Thor vs Hulk
Superman vs Gog (second death of Superman)
Galactus vs The Inbetweener
Professor X vs Shadow King
Batman vs Bane
Spiderman/Batman versus neo-conservatism in Reign/Dark Knight Strikes Back
Avengers versus Defenders
The Executionor's last stand
Xavier, Gambit, Wolverine, Pietro, Jean, and Rogue versus Magneto in that one right before he becomes Joseph.
Lucifer versus the Basanos
Morpheus versus Dr. Destiny
Dr. Strange versus Death (the first time)
Earth X Saga: Captain America versus the Skull; Tony Stark in the Red Ronin Armor, Namor, Loki and the Asgardians, and Galactus versus the Celestials; Magneto and the Silver Surfer versus the Absorbing Man
The Justice League and the ENTIRE WORLD (and some of Heaven) versus Mageddon
Swamp Thing versus a fear demon in Alan Moore's...sixth issue?
Honorable mentions: Animal Man versus the concept of irony in "The Coyote Gospels," John Constantine versus cancer, the Doom Patrol's first encounter with the Brotherhood of Dada and their battle with the Scissormen, Shade the Changing Man versus the American Scream.
I forgot which issue it was, as most of my books are buried in boxes (I just moved) but it's the fight that Superboy was involved in, and Superman fought him while he was on his parents farm... you know the book that he gets speared and is injected by liquid krptonite? hmmm... Arthur Adams does the cover art for the arc.