This is a virtually impossible question to answer. If the heroes form teams, like in Civil War, then you've got a bunch of different heroes all fighting each other and their "Fought Hero" counts are very similar. And with all the different heroes it's too difficult to figure out which one of them has a slightly higher count than the others.
I assume we're talking about Earth-616, since you said alt universe don't count. Though if they did, then "Deadpool kills the Marvel Universe" and "Old Man Logan", in which one person kills pretty much everyone else are pretty good contenders.
Ultimately though, short of reading every comic of every hero and keeping a tally of all the different heroes they've fought, there's no way to really know. To be perfectly honest, I daresay that every hero has fought every other hero at some point during their storyline, so they've probably all fought each other anyway.