I voted Imp... but I don't think I like Imps any more than I like Demons or Spectres... idSoftware simply had the greatest designs in all of video games.
I personally disagree on Doom3 having dropped that, as I loved seeing more believable monsters with abstract deadly appearances, rather than seeing floating tomatoes with horns, or the fantasy style of the Baron of Hell. What Doom3 did was make the monsters match the theme of the game, which is science fiction. My opinion on scaryness is that the classic ones aren't scary at all, the Doom3 ones are. When I myself first played Doom I loved the designs because they were funny, and cartoony, and kickass, but not scary.
But it's just an opinion, and I only mean that for Doom3. I love ALL the original Doom monster designs, and I wish they would just re-release classic Doom with 3d models and nice textures, then maybe even make a sequell to Doom2.
The classic Doom is, and always will be, the best