Odin vs. Thanos was one of the best. No PIS or CIS got involved, and both gave it their all. I know Thanos fans still carp about the ending, but it was a great display of the Titan's fighting spirit.
Any fight between Superman & Captain Marvel is great.
Batman vs. The Mutant Leader from The Dark Knight Returns - Round 1 & 2.
Virtually all the most powerful heroes(coming down to Earth-2's Superman) vs. The Anti-Monitor in the final issue of Crisis On Infinite Earths.
Punisher's first encounter with the Russian - brutal yet funny.
Odin's first modern age standoff against Surtur, from Walt Simonson's THOR days.
Whenever Thor goes all-out in armour & belt of strength, to challenge a Celestial like Exitar.
Wolverine's fights against Deadpool - amusing because Wade annoys him so much.
Thanos vs Thanos (only Thanos can take out Thanos)
Thanos vs Tyrant
I've always been mighty fond of the Thanos vs Beyonder fight.
Galactus vs Mephisto
Galactus vs Sphinx
Black Adam vs JSA
Black Adam vs Hawkman was pretty exiting.
Batman's fights with the Seven Deadly Masters after he got his back broken.
Batman vs Shiva (all 20 of them...)
Batman when he entered the air and fought KK out of his Batman's standard fighting stage... if not just for the impressiveness of it.
Although people don't like the writing (whine), I liked Superman vs Darkseid in Apok Now.
Superman vs Doomsday DOS
Superman vs Hunter Prey Doomsday
Thor vs Celestials
Thor vs Perrikus
Kingdom Come's ending
KORVAC!!!
Galactus vs. In-betweener. YES! This is how a cosmic fight should play out. Galactus and In-betweener grow to the size of entire planets and fight one of the most epic cosmic battles of all time, after they get sucked in through a massive black hole and In-Betweener gains the upper hand, In-Betweener is outsmarted and gets taken away by Lord Chaos and Master Order.