Traditionally, I've always answered Merckx to this question, even though he'd retired before I even learned how to ride a bike, so that's based on reputation alone.
However, it was Lemond who really got me interested in cycling. Sure, there were Hinault, Fignon, Kelly, Delgado, Abdoujaparov and Millar around at the same time, and they were all great, but my abiding memories of watching the Tour de France on C4 in that era are of Lemond.
Plus, in light of recent events, I've come to admire him even more as a person, as well as a cyclist.
d.