I gotta say, I chuckled a little at your statement "some of the fugliest I've ever played in any WAD ever (looking at you, MAP21.)" lol
Aesthetically I generally like Doom's levels for being more thematically consistent. Doom II has some fantastic-looking levels, as well as some of the fugliest I've ever played in any WAD ever (looking at you, MAP21.) Any semblance of thematic consistency in Doom II especially falls apart in the Hell levels, which include too many maps that just feel like weird starbases. The one thing in this regard I do like about Doom II is its use of large spaces which Doom couldn't do. I know that the Chasm gets a lot of hate but I always thought it pulled off a nicely ominous, decaying atmosphere. I don't even mind all that SFALL everywhere that much. MAP29 does this even better, and I don't think any other map in the two episodes left quite as much of an impression on me when I first played them. I also like Doom's textures better overall, for both the wider variety of techbase textures and all those cool half-tech, half-hell textures that were left out in Doom II for some reason. Doom II's new textures are a mixed bunch, and though the city/stone textures are useful in mapping they're too overwhelmingly brown.
As far as gameplay goes, it's a tougher call. The new monsters do add a lot to Doom II, and even in nasty-looking levels like MAP16 you get some nice intense action of the sort Doom doesn't have as much of. I like the more tight and often trap-heavy action in Doom, but I'd say overall Doom II simply benefits from having more variety of possible and realized battle scenarios.
As for a final verdict? Maybe it's nostalgia, maybe it's shallowness, maybe it's just because I get sick of brown after a while, but I'll have to go with Doom anyway. Doom II had some great ideas, but didn't see them out as consistently as Doom did. Ultimately, Doom II's greatest contribution to Doomdom is the possibilities it expanded for mapping with its expanded bestiary and added SSG and megasphere.
I agree with what you are saying, but unfortunately I'm just not really a fan of the Hell theme. I'm more into techbases, cities, and outdoor environments. I think Doom 2 went a little overboard with the Hell themed levels.
The new super shot gun as well as the new monsters made the gameplay so relatively good though.