The Clash:
Smartest: Joe Strummer
Dumbest: Terry Chimes (Topper Headon probably pretty close)
Toughest: Paul Simenon
And well, getting back on topic, CSNY:
Smartest: None of the above
Dumbest: Stephen Stills (Seriously, if Cros had three good years, Stills had, like, two good songs.) (And Nash is actually a pretty talented photographer.)
Toughest: Neil, but you knew that.