According to a poll on planet rock radio anyway they are...

Best singer

1. Robert Plant (Led Zeppelin)
2. Freddie Mercury (Queen)
3. Paul Rodgers (Free, Bad Company)
4. David Coverdale (Deep Purple, Whitesnake)
5. Ian Gillan (Deep Purple)
6. Bon Scott (AC/DC)
7. Ronnie James Dio (Rainbow)
8. Stevie Nicks (Fleetwood Mac)
9. Roger Daltrey (The Who)
10. Bono (U2)

Best guitarist

1. Jimmy Page (Led Zeppelin)
2. Slash (Guns N' Roses)
3. Ritchie Blackmore (Deep Purple)
4. Jimi Hendrix
5. Angus Young (AC/DC)
6. Gary Moore
7. Brian May (Queen)
8. Joe Satriani
9. Steve Vai
10. David Gilmour (Pink Floyd)

Best bassist

1. John Paul Jones (Led Zeppelin)
2. John Entwistle (The Who)
3. Chris Squire (Yes)
4. Phil Lynott (Thin Lizzy)
5. Geddy Lee (Rush)
6. Jack Bruce (Cream)
7. Steve Harris (Iron Maiden)
8. Lemmy (Motorhead)
9. Geezer Butler (Black Sabbath)
10. Roger Waters (Pink Floyd)

Best drummer

1. John Bonham (Led Zeppelin)
2. Neil Peart (Rush)
3. Keith Moon (The Who)
4. Cozy Powell (Black Sabbath, Rainbow)
5. Phil Collins (Genesis)
6. Ginger Baker (Cream)
7. Ian Paice (Deep Purple)
8. Roger Taylor (Queen)
9. Dave Grohl (Nirvana/Foo Fighters)
10. Eric Carr (Kiss)

I'm sure everybody here will totally agree with all these choices ;)