For me the best has to be Kirby's Sub-Mariner vs. Iron Man battle in TTA 82. Kirby had like only a weekend to get it done since Gene Colan became too ill to do more than the first couple of pages, and boy was Kirby in the zone that weekend!

Other favorites include:

JB's Thing vs. Thing in MTIO 50

Conway and Perez's JLA & JSA vs. Secret Society of Super-Villains in JLA 195-197.

You know, the reason I put a fight into greatest slugfests has to do with a specific punch thrown. The way it's drawn makes it unforgettable, like JB's Doc Samson doubling the Hulk over with one in the breadbasket in HULK 317, or Ben Grimm of 1962 decking Ben Grimm of 1979, first punch landed, in MTO 50.

Or one of the most unforgettable blows ever, Iron Man's "rabbit punch" to the back of Namor's head during the melee from TALES OF SUSPENSE, the Kirby part. I didn't know what a "rabbit punch" was, and still kind of don't. I assume it's any kind of clubbing shot to the back of an opponent's head or neck. Anyway, the old fists-together rabbit punch is fabulous. Every single punch in that comic is amazing. I can't remember who did the inking on that issue...