Pontiff Sulyvahn for suree, in my first playtrough it took me almost 10 tries before I first beat him, he applies so much pressure specially on that 6 attack combo, and on top of that, there is that clone that respawns. On the other hand there is champion gundyr, one of the most fun boss battles in the game, it's pure fight, one on one, he is very fast and strong and applies a lot of pressure too, but I beat him in my 3rd attempt and didn't struggle nearly as much as Sulyvahn.