Have to agree about Ornstein and Smough. It's one of the few boss fights that I really look forward to despite the fight itself being quite tough.
I'll also throw in Belltower Gargoyles, Gaping Dragon, Quelaag, Sif, Gravelord Nito, Artorias, Manus, Kalameet, and Gwyn. (The order in which I fought them)
For Demon's Souls, I remember enjoying fights like the Tower Knight, Dragon God, Old Hero, Lady Astraea, and the boss before the "final boss".
Haven't played Dark Souls 2. Although the Looking Glass Knight fight seems pretty cool. It's kind of like the Old Monk fight in Demon's Souls.