I'd say Unreal followed closely by Doom and then HL1.
Unreal itself was such a stupidly advanced engine for it's time, that very little could be compared to it. Even looking back at it now, it had superb music (I've got all the tracks!), stunning visuals and the craziest AI seen since Descent.
The game overall was just too long though and became dull towards the end. It was a superb achievement from a game engine PoV though.