I prefer the imp from Doom 3 to be honest. I think that the imp from the original Doom doesn't look menacing at all, same case with the pink demon and lost souls.
However, I think the originals are better games. I still play Doom 2, I'm trying to beat the Plutonia levels on ultra violence starting each level with just a pistol; the gameplay is so fast and the guns so satisfying, its crazy, and I'm not getting hit by nostalgia either, as I discovered this game just a few years back, around the time Doom 3 was released.
Doom 3 on the other hand was tedious to me.
So, uhm... yeah, I prefer the imp from Doom 3...
IMO, gameplay-wise, Doom 3 is lacking something, also if compared with Quake 1 or 2 or Half-Life IMO.
it is sometimes almost like a wave of not-very-goodness came and befell the FPS genre.
graphics have gotten better, gameplay, not so much...
it is almost like me first encountering Quake 3 Arena and being like "what?".
I generally preferred games with single-player gameplay. well, people who like deathmatch liked it, and it itself led to its own branch of the FPS genre (games primarily defined by multiplayer and DM + CTF gameplay).
Doom 3 seemed to be more like a whole lot of "I can't see crap" and monsters popping up behind the player, which I personally found kind of annoying (at least in the prior games).
also slightly annoying was the sound design, which basically seemingly equated "dramatic" or "exciting" with "noisy". ("what do we need here? oh yeah, a whole bunch of screaming and some baby noises...").
and, sadly, for newer FPS games, Doom 3 is still not doing that badly.
if I could see stuff, and it was less annoying, and if not for it being a good part of a decade before I had hardware powerful enough to get reasonable frame-rates with it, I may have been happier with it.
also thought Half-Life 2 was ok, but still kind of weak in some ways vs Half-Life (like, in HL, a person could wander around a bit more within each area, rather than everything being essentially a long unidirectional corridor).
in older games, you often had to find the exit, rather than essentially just going forwards until arriving at the exit (for pretty much the entire game).
elsewhere, much of the rest of the FPS genre has become dominated by Halo and COD clones...
dunno, could be nostalgia for all I know.