Flea
Les Claypool
Jason Newstead
Krist Novoselic

Although Newstead is not even close to the level of Flea and Claypool, Metallica just hasn't been the same without him As well, the late Cliff Burton of Metallica was awesome, too.

Likewise with Novoselic, Nirvana wouldn't have been Nirvana without him

And for those who don't know, Flea's actual name is Michael Balzary (just thought I'd throw that little tidbit of info in there) He's my alltime favorite.