This is gonna be tough....here's my picks
1) The original GDQ mods but especially G1: Steading of the Hill Giant Chief. I remember seeing this on the shelf right as it was released and saying "What the heck is a
Module?...and for that matter,what the Heck is a
Steading? Shortly thereafter I was run through said module, and I was hooked...this was ALOT better than the adventures other people had made up...we had to think, be stealthy, fight, parley, run...and at the end...it wasn't the end....continued in G2...talk about anticipation!
2) Moldvay/Cook era B/X sets: Clear and to the point . Good organization. Brought in alot of gamers, and supplied many years of fun. Still my all time fave version of the rules (not perfect, but darn good)
3) B2 The Keep on the Borderlands. An entire mini-campaign. Great tool for novice DMs, nice little side-treks (Mad Hermit, Lizard Man Mounds, etc), Home base filled with adventure hooks, and the Caves of Chaos themselves. D&D in it's purest form AFAIC.
4) T1 The Village of Homlett..much like B2, Homlett was a mini-campaign setting in a handful of pages...Gygax did a better job describing a big region of GH in a few paragraphs, than subsequent authors would do in entire books...Very cool
5) TSR, The Dragon, & Dragon mags up 'til about 1985 or so...great products in the overall sense
Honorable mentions for A1,2,3,& 4, U1,S1 & S2,B1, Brown Books & supplements, 1E FR grey box, FR5, Planescape boxed set, From the Ashes, 3E FRCS (don't like all the changes, but that is the benchmark for campaign setting books), 3E MotP, Of Sound Mind, Crucible of Freya, Rappan Athuk series,PGttW, Monte's AU, Mutants and Masterminds, Midnight, and last but not least, the Epic Level Handbook...HA! Just kidding