Sunless Citadel is pretty imaginative, the latter ones less so. Personally, I would choose the Rappan Athuk series (the third and last part is due out in May). It has several good points:
-it is large enough to accomodate months of playing, but not
too large - there is a definite tight feel to the levels and, although clearing the thing out is impossible, it is an accomplishment when you explore one completely.
-the levels are clearly themed and imaginative. Examples include "The Gates of Hell", "Lair of the Spider Queen", "The Upper Temple of Orcus", "Halls of the Phase Minotaur King", etc. RA also uses the old convention of main and side levels. Many of these are hard to access, subtly reinforcing the exploration theme.
-Deadly, but not
completely deadly. Players can triumph if they are well organized, but if they make foolish decisions ("going down the Well" being the most foolish), they are going to be dead.
-The writing is nice - not overwritten, but definitely enough to help a DM describing the various chambers.
-Support: you can download two wilderness expansions, new maps (the originals were pretty crappy), one colour wilderness map handout for your players, also prestige classes, etc.
Questionable points:
-There are a few rule mistakes in the first one, as most of it was written before the release of the 3e MM and DMG. Nothing too bad, just an undead having a Con score (giving it +30 Hp) and a few magic items absent from 3e. Your players will hardly notice any of this.
-Some people hate the indestructible monsters (like a certain "Dungie"), the sheer deadliness and the lack of a carefully detailed dungeon ecology. These people don't belong to the target audience, and this is clearly written in the introduction. Granted, I have never been an adherent to the "Guard Room#30" school of adventure design...
Oh, and it is assumed that you have 6 characters instead of the standard 4...
-Bad maps - but you can download and print some really beautiful ones.
Other good dungeons:
-Dark Tower from Judges Guild is a classic, and rightly so. Unfortunately, the conversion to 3e is really bad, and it is a nightmare to use thanks to organization and the fact that 3e plays a bit differently than 1e (certain encounters will be easier, others much, much harder).
-Tomb of Abysthor: I haven't heard anything bad about this one. I haven't seen it yet, either, but it is supposed to be very good (levels 3+).
-NeMoren' Vault: ditto. Got rave reviews, might be hard to find now.
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