Top guitarists
Diamond Darrell - Pantera (rhythm/lead/noise) - the one and only. Noone I know is as versatile and mean on the guitar as Darrell.
James Hetfield - Metallica (rhtyhm) - master of thrash rhythm guitar.
Jerry Cantrell - Alice in Chains (rhythm/lead)
Stevie Ray Vaughan (whatever...) - bluesmaestro, who knows what he might have achieved if he had not been killed...
Jeff Hanneman and Kerry King - Slayer (rhythm/lead) - we're talking speed here.
Jimi Hendrix (everything) - the father of rock solo guitar.

Top singers
James Hetfield - Metallica
Phil Anselmo - Pantera
Lenny Kravitz
Layne Staley - Alice in Chains
Freddie Mercury - Queen
Kurt Cobain - Nirvana

Top drummers
Vinnie Paul - Pantera
Dave Lombardo - Grip Inc., former Slayer-member
Chris Contos - formerly in Machine Head, now in Testament.
Igor Cavalera - Sepultura
Paul Bostaph - formerly in Slayer, during the short time he was member of the group, he did some awesome stuff.

Top bassists
Flea - Red Hot Chili Peppers
Cliff Burton (R.I.P.) - ex. Metallica

Top maniacs
Kurt Cobain - Nirvana
Billie Joe and the rest of Green Day
Diamond Darrell - Pantera
Beastie Boys
Jimi Hendrix

Top lyric writers
James Hetfield - Metallica
Phil Anselmo - Pantera
Robb Flynn - Machine Head
Max Cavalera - Sepultura
Jerry Cantrell & Layne Staley - Alice in Chains