Of the old school thrash bands still playing that are somewhat relevant, here is the order of their popularity:
Megadeth/Slayer (a tie, Slayer is maybe more popular now, but back when the bands sold a boatload of records Megadeth was all over MTV, radio... its about even imo)
Testament (Not as popular as Anthrax, but easily more popular than the others beneath them)
Exodus (More popular than Overkill due to more exposure on Headbangers Ball back in the days and Holt recently playing with Slayer has had to give them several new fans)
Overkill (Overkill has become as popular as ever due to their last few albums being critically acclaimed in the press and That Metal Show).
1-13 Philip H. Anselmo & The Illegals