DCC 1 Idylls of the Rat King (1-3) gets my vote for best.
Other good ones are:
DCC 44 Dreaming Caverns of the Duergar (1-3)
DCC 46 The Scaly God (4-6)
17 Legacy of the Savage Kings (4-6)
10 The Sunless Garden (6-8)
5 Aerie of the Crow God (7-9)
30 Vault of the Dragon Kings (10)
13 Crypt of the Devil Lich (15)
Honorable mention: #35A Halls of the Minotaur(from Gazeteer of the Known Realms boxed set) - for 0 level characters. The only reason this isn't #2 on my list is it comes as part of a boxed set (which is totally worth buying, but the adventure is a very small part of it).
There are many others that are really good, but if I include them, it would hardly be a best of list anymore.