That's a good question, actually.
Neither Doom nor Unreal 1 had engaging storylines and were both really breakthroughs in engine development more than storyline or gameplay.
Doom was the first FPS I ever played. It did have a certain immersiveness to it -- Half Life is really the only game that came close to how frightened Doom could make me when I played it. But then, understandibly, it had one of the most simple gameplay schemes around. Kill a few enemies, get a few keys, kill the main boss, yada yada. That's one of the things that worried me when id announced they were making a Doom 3 -- whether or not it would live up to the "Doom" name because of the nature of the gameplay for the previous two Dooms, which was fine for it's time, but I doubt it could really compete with singleplayer games with better immersitivity and gameplay.
Although I'm not as anxious about Unreal 2 being good as I am about D3, U1 isn't proof enough that U2 will be an innovative and immersive singleplayer experience. U1 was a breakthrough, like Doom was, graphically more than any other way. It had some decent singleplayer elements.. but the lack of monsters to tide me over until I figured out where I was going made it a bit boring. After playing online UT for a while, the monsters in U1, while probably more advanced than any AI before it, were pushovers compared to humans.
Anyway, I guess I'm undecided as of yet; I'm probably biased to Unreal 2 though, for obvious reasons.
Only time will tell.