I mainly use the one Kambei mentioned. They also have a Midnight Blue that works pretty well as as dark purple.
I use Tamiya Purple for doing Alien flesh tones, but a lot of the purples I've bought just don't do it for me --- too Sputnik-y, I guess
Actually, Lace Spudneck is a good one to ask, since he uses the hell out of Purple for his Mindbender & Tinky-Winky customs....