IMHO, the best unofficial supplements are:
Lords of the Night: Vampires, Lords of the Night: Liches, Torn Asunder: Critical Hits, The Starfarer's Handbook (for Dragonstar), Dweomercraft: Familiars
Setting wise:
Redhurst: Academy of Magic, 7 Cities, The Book of Taverns (all of these are locations you can plug into your game). I also liked Freeport before it got campy.
Adventures:
The Hamlet of Thumble, The Village of Oester, some of the Necromancer stuff (Grey Citadel, Vault of Larin Karr, and their Mr. T book, or so I've heard)
For many other companies, it really depends more on the author, not the company, because most of the bigger companies use a lot of freelancers. For instance, I've liked most of Sam Witt's stuff. Most seem to like Mike Mearls stuff (I'm more of an 'eh' on him)