Top three

I think at some point I did top 25 lists for hard rock/metal vocalists and drummers, but never got to bass players and guitarists. For now I just give you my top three for each instrument.

Vocalists
1-Rob Halford-Easy pick, the range and the power for such a long time.
2-Ronnie Jame Dio-Amazing and highly consistent over the course of a very long career plus he has been golden with every band that he has been with.
3-Bruce Dickinson-Incredible voice, he had added another dimension to Maiden when he joined.

Guitarists
1-Hendrix-He did more in his first album than other top guys have done in twenty years of work. Plus it all just seemed to come so easy for him
2-Tony Iommi-He didn't just play heavy, he created and defined heavy metal guitar.
3-Adrian Smith/Dave Murray-Yes, they go as a pair. Incredible players who just seem to be pulling out a nearly endless supply of riffs

Bass players
1-Steve Harris-No doubt on this one. He has those fingers flying and he doesn't even look at them. Fantastic.
2-Geddy Lee-Just interesting bass parts right from the start of their career and then he just got better.
3-Geezer Butler-Killer sound, just bending those parts and like Iommi he knew that the tone was more important than the speed.

Drummers
1-Neil Peart-Just all over the place and he adjusted his style to suit the many sound changes of Rush and he still sounded no matter what he was playing.
2-John Bonham-Superb rhythm and style and he grew as a player as well.
3-Bill Ward- The most underrated member of Sabbath. On those early albums he was destroying those drums, a big heavy hitter who along with Butler really laid
down that heavy backbone for Sabbath's prime albums.

Keyboards?
Well, Jon Lord is undoubtedly the best organ/keyboard player in hard rock/metal, he was just everywhere and really added such depth to those classic Deep Purple albums. Other than him I can't think of a whole lot of keyboard players who were that outstanding. Some of the guys for Uriah Heep had some moments and Tony Carey was fantastic on Rainbow's Rising, but not a lot of lot people who really stand out that much.

There you go.