The Immortal for me, always makes the think back about the amazing last 3 bosses where we kept screaming and shouting. Had the whole melee in iceblock on KT with like 15 people screaming like a girl but noone died, last really strong nerdscreams I have heard in this game and it was a while since KJ.
If talking about titles overall I'd say Scarab Lord, and yeah the legit one, now those people were like the uberbosses of the realm. Shame the title got more or less ruined.