I'd plump for Lemond as his story has absolutely everything. It was just a few years before I got into cycling, but it's the best story out there in my opinion.
He is certainly the greatest American rider there's ever been. I say that as someone who's first poster of a cyclist on their wall was Lance Armstrong winning the World Championships when I was 16. I discount Armstrong now though as I find it unthinkable that he didn't dope his way through his career. (Read 'From Lance To Landis').
Anyway, Lemond was the darling of American cycling as youngster. Sports Science was in it's infancy, but when tested by American Scientists he gave remarkable results. He showed the highest VO2 max ever recorded by a cyclist (bigger than Indurain later recorded with his huge lung capacity!) and it was thought that Lemond could dominate the Tour. He was simply off the scale.
He won his first Tour in 86, though could have won the year before but was ordered to ride for his captain Hinault, dropping back on a mountain stage to ride for Hinault, who won that year. Then in 86 Lemond was made co-leader with Hinault, but the duo raced mainly against each other as Hinault constantly attacked - even when Lemond was in yellow, and he had to be chased down by his own team mates.
He was the first American to win the Tour.
In 87 he was shot in a hunting accident. It took him two years to recover and he carried shotgun pellets around in his body ever since. This would later impact on his ability, as the lead from the pellets took its toll.
However this did not stop him winning the Tour two more times, most famously by only 8 seconds over Laurent Fignon on the final time trial stage of the race in 89 - he was riding for a small team with barely any team strength and was not expected to challenge. This race is also interesting because Lemond used TT bars in the final time trial, no one had really done this before.
I think you could go on and on with his story. But he's interesting because he was clean, he was the first American, he was a unique physical specimen, he brought Srort Science and new technology to the fore of cycling and he even became an expert in Sport Science himself later in his career; helping him as a leading voice against dopers in the sport.
In the world of could ofs and should ofs, Lemond could have won 6 or 7 TdFs himself easily, if there is such a thing as an easy Tdf victory!