That is impossible to answer.
To get at the heart of your question compare the palmares:
Lemond,
Armstrong.
Lemond has a decent palmares from top level European events that span the entire season and his entire career. LA has a few good results in Europe including a classics win but most of his results prior to 1999 are from NA races which is clearly a lesser accomplishment. Once he started focusing on the TdF that was pretty much all he rode outside of a few races in preparation for the TdF.
Look at Lemond in '86; He wins the TdF, is 4th in the Giro, 2nd in Milan San Remo as well as other results. In '89 he wins the TdF (including 3 stages) and his SECOND World Championship! He wins the TdF again in 1990 but also has a 2nd in '85 and a 3rd in ' 84 at the TdF as well and a solid palmares in other major European races throughout his career.
Lemond was politiced out of one TdF win and had a decent shot at another but the TdF is only one race. Combined with his other results, spread across a COMPLETE season he is, and has always been, the greatest American cyclist to date.
Edit: And oh yeah, all that stuff he said about LA, all that stuff that cost him so much, he was ****ing right all along.
Lemond ftw.