I don't think I could narrow it down to just one person for most of these categories, but I'll try to limit myself to no more than two or three. I'm a huge Dream Theater fan, so I have a natural bias towards those guys, so I'll try to keep them out of this - they would all be within my top 3's anyway.
Guitars: Eddie Van Halen and Steve Vai
Keyboards: Derek Sherinian and Neal Morse (I haven't heard much Morse at all, but I love what I've heard so far)
Drums: Neil Peart and John Bonham
Bass: Geddy Lee and Pete Trewavas
Vocals: Freddie Mercury and Steve Hogarth