The best classic modules, ranked.
First, the answer to the question. What is the best (single) classic module?
....X2, Castle of Amber. It has everything. It's a B/X module, but can play fine with AD&D characters. It has a loose metaplot/mystery if you want to emphasize that. It has multiple different areas to explore. It has combat, traps, and classic "player skill" puzzles. It has dungeon and overland. It has madness and Lovecraftian horror. It has a whole demi-plane subplot. It is chock full o' stuff and a little bit gonzo.
...it is is messy and wondrous and perfect.
But I'd give an honorable mention, on the AD&D side, to WG4, The Temple of Tharizdun.
Some others to look at:
A Series (A1-4) The slaver series has lost a lot of luster with modern gamers because it is considered rail-roady, but it's good fun!
B1, B2, B3 All of these all, IMO, a little overrated. Still fine.
B4 Imagine how much better the world would be if this module, and not B2, had been included with all the basic sets!
C1, C2 Both classics; now that Tamoachan got a WOTC reprint, it pays to use Inverness.
EX1-2 The Wonderland modules.
I Series (I1-I6) I would argue that this is the best series of AD&D modules ... and if you argue with me, I will FIGHT you.
UK1 Perhaps the most new school of the old school modules.
X4-X5 Isle of Dread gets the hype, Castle of Amber gets the top prize, and other "series" that became bloated (I am looking at you, Temple of Elemental Evil and GDQ) are still beloved, but this weird little double of Desert Nomads and Temple of Death is the best module series / proto-AP TSR ever made.