You guys are all noobs.
The hardest Death is definitely CV1's. And don't tell me he can die easily with the Holy Water trick. Any boss is easily defeated if you exploit them. And any boss in Ecclesia is easily defeated if you use the dark/light union with Death Ring(s). What matters here is that the bosses be defeated using normal methods, like a man.
Also, since when was Dawn of Sorrow hard? The bosses were just average in difficulty (though most of them pretty awesome). Symphony of the Night's Death is also terribly easy, and so is Super Castlevania IV's for having a dreadfully predictable pattern.
Julius is actually legitimately difficult, but you can beat him on the first go if you stop playing with your penis and start using your hands. Ecclesia's bosses are a lot harder, specifically Dracula (except Arthroverta, but then again, lol Arthroverta).
And finally, the easiest Castlevania game is Simon's Quest (no, it's just confusing, not hard), and the hardest one is probably Haunted Castle or Vampire Killer. Haunted Castle's bosses are possibly the hardest.