The earliest D&D adventures
Although there were D&D adventures published before the Giants, they did not have a wide enough distribution to really be worth considering except in passing. Members of the Acaeum know more about them than I, in any case (and perhaps Grodog might pass by). The Temple of the Frog (from Supplement II: Blackmoor) is also an oddity that lies outside the normal consideration of adventures.
The First Year: 1978
G1: Steading of the Hill Giant Chief (8 pages)
G2: Glacial Rift of the Frost Giant Jarl (8 pages)
G3: Hall of the Fire Giant King (16 pages)
D1: Descent into the Depths of the Earth (20? pages)
D2: Shrine of the Kuo-Toa (20 pages)
D3: Vault of the Drow (28 pages)
S1: Tomb of Horrors (12 pages + 20 page illustration booklet)
B1: In Search of the Unknown (28 pages)
Seven of the first eight adventures for D&D were written by Gary Gygax, the other (B1) by Mike Carr, and all are remarkable in how brief they are. In truth, G1-3 and D1-3 were all part of the same series (which would be finally completed in 1980 by Q1) and would be collected in later printings.
G1, G2 and G3 are linked by a simple theme: slay the giants. This is an archetypal theme that owes much to Norse mythology, and resonates well. G1 is simple in construct, but as the adventures progressed, Gygax added more challenges and interest to them. New enemies were introduced, terrain features were varied, and the tactics of the giants grew more cunning.
These early adventures required skill from the DM to turn them into more than just hack'n'slash fests, but the material was there. G3 was the most interesting, as it introduced not only the treacherous dwarf Obmi and a strange underground temple, but also one of the signature races of D&D, the Drow.
D1 and D2 began another archetypal quest: Discover the strange things that live beneath the surface of the earth. Where the other modules were somewhat limited in scope, the Descent literally covered miles of what much later became known as the Underdark. Even more than G1-3, these relied on the DM's invention to fill in the blanks. However, the environment presented was vividly described and enough detail was given for the inventive DM to really make them work.
D3, however, being the sixth part of this series, was extremely unusual. Less of an adventure than a description of the destination of the adventurers' pursuit through the Underdark, the Vault of the Drow was a potentially dangerous place full of webs of intrigue. Of all the adventures ever published for D&D, this is the one that requires the most from the DM. It is also one the most influential on the mythology of D&D - both in Greyhawk and in the Forgotten Realms.
G3 (1978) and D3 are the two great classics adventures by Gary Gygax for AD&D. The potential provided by those adventures would not be equalled in his later releases.
S1 is a nonesuch: an adventure designed to kill the PCs of people who think they're better than they are! It is similar to the classic dungeon adventure with one exception: very few monsters and many, many traps. The variance in ways that your PC could die during this adventure makes it deserve its classic status.
B1 is also unusual: it was designed for novice DMs. A backstory was given, and the map was drawn, along with descriptions of some rooms and their special features. However, not everything was filled in! The DM had to stock the rooms with monsters and treasure, as well as other interesting items. This style of dungeon would only be attempted once more, in B3, but that product was withdrawn and rewritten before wide sales, so B1 also stands alone. I don't think that by any measure it can be considered a great adventure, but it was the start for many players.
Cheers!