Well, this is a REALLY hard question..
Obviously, Opeth is my dream line-up but, otherwise...
I guess it depends on what kind of band I want:
For something really heavy (and still good):
Drums: Toss up between Gene Hoglan, Martin Lopez, Hellhammer, and Joey Jordison (bash Slipknot all u want, the man is a fuckin' MACHINE)
Rhythm Guitar: Devin Townsend
Lead Guitar: Probably John Petrucci, or maybe Carl August Tideman
Bass: ICS Vortex, or Steve DiGiorgio
Keys: If it had keys, Steiner Sverd Johnsson
Vox: Possibly Karl Middleton, Peter Dolving, or maybe Isahn. Or Mike, clearly.
For something less heavy, but still kinda heavy:
Drums: Martin Lopez, no question. Except for maybe this session drummer called Matt Chamberlain.
Bass: Chris Wolstenholme
Keys: Tori Amos
Rhythm guitar: Anders Nystrom, Matt Bellamy
Lead Guitar: Mike
Programming: Steven Wilson
Vocals: Steve, Mike, Matt and Tori can do harmonies
, with maybe Chris Cornell, Jonas Renske, Maynard James Keenan or just Mike on lead vox.
I'm sure I've forgotten lots of people I shouldn't have...