Mother's Milk and Blood Sugar Sex Magik are still my personal favorites, followed by The Uplift Mofo Party Plan and maybe Freaky Styley.
I saw them play a spring break concert in the early 80's, and they brought on a unique funk/punk groove that no one else had at the time. The energy level is still there when they play live these days, but the studio production is a bit too polished. The later albums were fine for big radio play, but the best magic was recorded in the early part of their history, IMO.