Hey, c'mon, it's like a rite of passage for new forum members, and I like exercising my brain, running through the old-school maps once again.
Doom the First, it's an easy E1M3, E2M2, E3M6 and E4M6, with E1M3 being the best in regular Doom I and E4M6 the best in Ultimate Doom;
Doom II, The Living End hands down (with Abandoned Mines a close second).
Plutonia, either Odyssey Of Noises for it's ambitious hugeness, or Go2It for the sheer insanity.
And TNT...... you know what? I'll go Deepest Reaches