Top Five, eh? Well, in the order they pop into my head:
City of Greyhawk Boxed Set - a wonderful resource, lots of poster maps, adventure cards and ideas, personalities, places and gods, all crammed into two 96-page books. Still useful for me as a city resource, 20 years after publication.
Tome of Horrors (Necromancer Games) - just pips the 2e
Monstrous Compendium into the list, this is a volume full of worthy nasties. It's great to pull something out of there that 3.xE players who have memorised the SRD don't expect. Brings a smile to grognards' faces, as well.
Griffin Mountain (Chaosium/ Moon Design) - HUGE campaign area, descriptions on citadels, personalities, rituals, Chaos Gods, hunting regions and a Mountain where Griffins lair. Glorantha comes alive in these pages, an area where culture is important (and not in a petri dish).
More info here
The Grey Citadel (Necromancer Games) - possibly the best city-based adventure I've read (though SCAP - when my H/C volume finally arrives - may top it), this module offers investigation, role-playing and a brutal dungeon crawl. Multiple plot lines dovetail together to a deadly resolution involving demons, blackguards and the local Thieves' Guild. The book delivers a city background which can be used for further adventure (or as a base of operations) on top of the adventure.
Earthdawn (FASA) - Fantasy/ Horror roleplaying, with a novel (and workable) step/ action dice resolution system, magic system which can have Consequences and some of the creepiest adversaries ever.
Current Earthdawn Edition site
Bubbling Under - Tunnels & Trolls (5th Edition) - first RPG rules I bought, fondly remembered, and with DM advice and world-building tips I still use today; D&D Cyclopedia - great rules compilation with material which a 3.xE game could easily cannabilise (Mass Cimbat Rules, I'm looking at you), it's a surprisingly sophisticated system; Shadowrun 2nd Edition; Paranoia 2nd Edition.
Tunnels & Trolls No links for the earlier Paranoia or Shadowrun editions.