lots of good stuff in this thread:
Extended F@mm - Happy Fuck You Songs
Deltron 3030 - Self-Titled
Del The Funkee Homosapien - Both Sides of The Brain
Jurassic 5 - Power In Numbers
Aesop Rock - Labor Days
Immortal Technique - Revolutionary Vol 1
Immortal Technique - Revolutionary Vol 2
Zion I - Mind Over Matter
Brother Ali - Shadows on the Sun
Tonedeff - Hyphen EP
The Streets - Original Pirate Material
Outkast - Speakerboxxx
Nelly - Nellyville
Cannibal Ox - The Cold Vein
Binary Star - Masters of the Universe
hmm, I guess all of those aren't "must-haves" in the truest sense, but if you have those you'll have an extremely well rounded collection full of good stuff. My personal "must-haves" on that list are eF@mm, Deltron, and J5.