Doom II is in some ways a quite a bit different from Doom I, in my opinion atleast. Doom1 had the perfect pacing, level structure, design, look and feel that made it what it is. If you notice, Doom 1 is dramaticlly slower in gameplay compared to Doom II, which is more of a balls-to-the-walls gun fight against crazy odds each level.
Personally, I'd say Doom I is the best -- no doubt -- but Doom II is still a great game.