Ah, yeah, that's the one I forgot about totally. Music!
I think Doom has the greatest musics of whole gaming history. And the greatest of them all is in Doom II map 27. It's just... fits perfectly for that level. Gotta' play it again!
In doom II there's many good levels like "Dead simple" and "Gotcha!"
(The legendary Cyberdemon vs. Mastermind level!), but I still prefer more levels of Ultimate doom. From there, my favorite is E3M1 "Hells Keep" if I remember correctly.