I've got 'Uncle Jam Wants You' and 'Ultimate Funkadelic' (compilation), both of which are OK, but not their best apparently. I also have 'First Thangs' by Parliament which is an amalgamation of their first album plus a load of extra tracks, and I LOVE IT! It's about as varied an album as you're likely to find, with funk, gospel and even a bit of country and western (well GC's take on C&W anyway). Buy it, it's fantastic. In fact, that reminds me that I must buy some more Parliament forthwith.
On a similar vein, get some Sly and the Family Stone - 'Stand' and 'Life' are both excellent, 'There's a Riot Goin' on' is also very good but more laid back.