I don't know who are the greatest, I'll just give my favourites
Guitarists:
1. David Gilmour
2. Jimmy Page
3. George Harisson
4. Slash
5. The Gipsy Kings
Drummers:
1. John Bonham
2. Michael Giles
3. Stewart Copeland
4. Carl Palmer
5. Michael Lee
Funny thing is I'd vote Led Zeppelin members number one of their category, except for the guitar.
Edit: I agree with you Docron, I could definitively have put Tom Morello somewhere in my list. And thanks Jarlaxe for reminding me of Mike McCready