I'd like to say The Evil Within; it provides an exhilarating amount of bosses with interesting designs and concepts, and they almost always feature some sort of gimmick that makes the battle more challenging; such as the poisonous gas with The Keeper, fire-only with Laura, the bear traps with The Sentinel or the WomboCombo of Two Brothers. Its biggest downside - other than some bosses coming back from time to time - is the final boss itself, which is unfortunately an almost scripted battle from beginning to end, and unfortunately kills the awesome streak of engaging boss battles.
Bloodborne's bosses with its Cthulhulian approach, Metal Gear Solid's with their crazy antics and personality or the simple but definable monster-ish battles with Resident Evil's bosses should take the crown.
