My faves:
S1- Tomb of Horrors: EVERY 1ed gamer (well, almost every) has uh... ahem... "horror" stories about this module, and will become quite animated talking about their experiences, and where they died (it wasn't a matter of "if", but rather "when" and "how"). I played it twice (and died both times) and DM'd it numerous times, and ALWAYS had a blast with it.
T1-4 Temple of Elemental Evil: Originally, this was 2 separate modules (The Villiage of Hommlet and the Temple. It will take characters from about 1 to about 8th level (so the book says). This can also be deadly, and can (if they are willing to pay attention) teach new players that it is possible (and a good thing many times) to run away and come back to fight at another time.
GDQ1-7 Queen of Spiders: This is another, like the Temple, where there are several modules all run together in a series and placed togethr in a "super module". This combined module includes the Giant series (G1-3), Descent Into the Depths of the Earth (D1), Shrine of Kuo-Toa (D2), Vault of the Drow (D3), and the motherload- Queen of the Demon Web Pits- The battle against the Queen of Evil, Lolth. This combined module will keep you playing for a few months. My original group played T1-4, A1-4, and GDQ1-7 in succession (actually, we played all of the original modules... not the compilations)... and it took us over a year (then we played the Tomb, and lost everyone we had kept alive for the full year).
A1-4 Scourge of the Slave Lords: A really great series where the characters are hired to find and attack slavers who are raiding a coastal town and stealing its residents, and finish up with the characters being captives of the Slavers (the end of the 3rd module), and escaping, and having to escape the island by using their wits, teamwork, and a bit of luck (but NO gear or spells). Overall, a really fun series. (spoiler... A4 (the escape) is heavy on the role playing aspect of the game... if that is not your group, then they might not like this.
The 4th one is proving to be a bit more difficult to decide. I am wavering between:
WG5- Mordenkainen's Fantastic Adventure
WG7- Castle Greyhawk
B1 -In Search of the Unknown (a Basic game module, and the first module I ever played)
C2- The Ghost Tower of Inverness
WGR1 -Ruins of Greyhawk
Honorable mentions (not the best avilable, yet still fun to play):
EX1- Dungeonland
EX2- Land Beyond the Magic Mirror