PCS ranking is useless. It heavily favours sprinters and stage winners. Viviani is in the All Time Top100 which should be charged as a crime against humanity. Please do not use that BS. Here is a much more accurate ranking:
https://www.cyclingranking.com/
Regarding the discussion, Van Garderen could probably be, in overall terms, the best american in the 2010-2020 decade, which is a bit underwhelming. He lacked individual wins in the international field - He is retiring with a Giro stage win, 2 Vuelta Catalunya stages, a Tour de Suisse stage, a bunch of US Domestic races and tons of near misses: Overall, he has 3 podiums at Dauphine, 1 podium at Catalunya and 2xTop5 in TdF.
I
n GT he was extremely dissapointing. Don't know if it was due to having low endurance / recovery or being unable to cope with pressure.
Horner is undoubtedly the one who got the biggest single achievement, followed by Talansky. Then you could probably slot Van Garderen.