Hard to choose one but i'd say, in no particular order : Senna, Mansell, Mcrae, Sainz, Mikola, Blomquist, Clark, Stewart, Vatanen, Loeb, Makkinen. All highly skilled and entertaining in there particular discipline and in their own right.

In addition to these i used to love watching parrish and sheene tear it up in truck racing.

What about adding bikes into it?
Schwantz, Doohan, Walker, Bayliss, Rossi, RON Haslam, Foggy, Lampkin