Lots of really good books listed so far. I'd add the Mutant Massacre from X-Men 210-213.
I think some people are looking for some more books off the beaten path that they may have never heard of so here are 4 of my favorite lesser known books with pictures:
THOR #385: BEST HULK vs. THOR fight ever!!! Period. I read this book 1,000 times as a kid. The inside is drawn by Eric Larsen of Spider-Man & Savage Dragon fame, but it's early work of his and looks awesome. Fight starts on pg. 5 and lasts the whole issue. At one point the Hulk throws a train on Thor. Great stuff.
CAPTAIN AMERICA #367: The "Acts of Vengeance" storyline. In this issue MAGNETO fights the RED SKULL for revenge (since Magneto spent time in the Nazi death camps). Red Skulls fate at the hands of Magneto at the end is the perfect ending. Awesome issue.
DAREDEVIL #249: WOLVERINE vs. DAREDEVIL fight. Basically, there is this villian committing hate crimes against mutants. Daredevil wants him to get a fair trial, while Wolverine wants to kill him. Very strong artwork.
AMAZING SM #274: This is one of my all time favorite Spider-Man issues, since it really gets to the heart of Spidey. Don't be fooled by the Secret Wars 2 crossover label. It starts with Mephisto and the Beyonder arguing over the value of a human life and whether or not the Beyonder should just destroy the world. They then make a bet that involves Spider-Man. Basically, Spidey has no idea this is going on. He over hears that someone is going to assasinate the Kingpin. So the Beyonder sends a demon to try and discourage Spidey from saving him. However, in the end Spidey values life, even that of his enemies, and does everything in his power to stop the assasination. Very cool issue that tells the story of how heroic Spidey really is.