My own recommendations for quality 2-session adventures would be (depending on party level and rules system):
U1 The Sinister Secret of Saltmarsh: Probably the best choice for a starting group, since both N1 and
T1 can become slow in the early going before they hit the dungeon.
N1 Against the Cult of the Reptile God: Can bog down to 3+ sessions if they spend too much time in town; otherwise, excellent for low-level characters.
A1 Slave Pits of the Undercity: Satisfying and very quick to drop into.
I2 Tomb of the Lizard King: One of my favorite dungeon crawls.
C2 Ghost Tower of Inverness: Like A1, but with more of a focus on puzzle-solving, and more gimmicky than reality-based; some groups may not like the flavor, but for what it is, it's superior.
X4 Master of the Desert Nomads: Diverse, challenging, quick-playing wilderness adventure; might need a bit of surgery at the ending if you're not moving on to X5. Some groups may not appreciate the railroading, though.
S2 White Plume Mountain: Can be finished off by a good group in 1 session, but it can kill the unwary quickly, too. Any extra time can be filled in with Dragotha's Lair if desired.
Since old modules require a fair amount of conversion time, it really doesn't matter if you go back to D&D, AD&D 1st, or AD&D 2nd; but if you're playing straight second edition, someone else can probably make better recommendations than I can.
