So many have already been mentioned; love most of the early Thing/Hulk matchups, and the Avengers vs FF in FF 25 and 26 were high spots.  In addition, though, I've always enjoyed Daredevil/Submariner from DD #7.  Beautiful Wally Wood art, and a great story that truly defined what a hero is, for me.

I'm going all Silver-Age for my choices, and several have been mentioned already -

Hulk vs. Fantastic Four [Fantastic Four #25] & Avengers [Fantastic Four #26]

Hulk & Sub-Mariner vs. Avengers [Avengers #3]

Sub-Mariner vs. Hulk [Talses to Atonish #100]

Sub-Mariner vs. Daredevil [Daredevil #7]

Thor vs. Hercules [Journey into Mystery Annual #1]

Spider-Man vs. the Sinister Six (Doctor Octopus, Electro, Vulture, Sandman, Kraven the Hunter & Mysterio) [Amazing Spider-Man Annual #1]

Avengers vs. Squaron Sinister [Avengers #70] & Invaders [Avengers #71]

I think someone hit on it upthread. The presence of the Hulk and his very nature as a character dictates his being involved in most of what we call "slugfests". I mean, in terms of "fighting", that encompasses gunplay and martial arts and even battles within the mindscape. Miller's Daredevil vs Gladiator.  Lone Wolf carving through assassins. Prof Xavier's mental "battles". Doc Strange's astral body against Mordo's.

But when we term "slugfest", then I think of battles in which fists are most prominently involved, and the Hulk redefines the fists and the superhero. If DC had cared to, they could have used Grundy as their resident Hulk in order to get in on the 70s Hulk Craze. I've never understood why DC did not do that. I mean, yes it's creatively corrupt to rip off the success of another company, but by the time the late 70s rolled around, Grundy was sitting there in the midst of the "horror"-themed explosion and, as far as I know, missed it.

I would have loved to have seen a Solomon Grundy Hulk-style comic done by, say, David Michelinie or Tony Isabella with art by Keith Pollard, Dick Giordano or Ernie Chan on inks?

well i gotta go with Superman vs. Doomsday
and Batman vs. the Mutant gang leader in "Dark knight returns"
Despero vs the(Giffen/Demateias) JLI, hell of a lot of city blocks got
busted up in that fight.
and an Army of Daxamites under Darkseid's influience vs. the Legion of Super
Heroes, (Great Darkness Saga anyone?!?!)

Journey into Mystery # 112, Jan 1965

I read it in a Treasury Edition.
Over. and Over. and Over.

#1 - Hulk vs Fantastic Four (Fantastic Four #25) & Avengers (Fantastic Four #26)

#2 Sub-Mariner vs Hulk (Tales to Atonish #100)

#3 - Hulk vs the Avengers (I don't know the Avengers issue number, but on the cover the Hulk is shown hidden underground beneath the Avengers.)

#4 - Ben Grimm vs Silver Surfer (The issue of the Fantastic Four where Ben flies into a jealous rage when he finds the Silver Surfer with Alicia)

Chad, DC DID get into the Hulk style stories on occasion.  Anyone remember Blockbuster?

Oh, and since no one else seems to have mentioned it, one of the most memorable slugfests was in FF 40, The Thing vs. Dr. Doom.  The Thing crushing Doom's gloves, in particular.