I'm just saying OP should just make a server of the pack they like, as it's just as effective as relying on DL numbers.The point you're making about people downloading and then not playing (long) is valid.... for all packs Which means that, yes absolute numbers could be lower, but percentage wise it wouldn't really matter
Chances are unless OP's taste differs from the norm what they like should mesh rather closely to what the mainstream likes.
You can't go wrong with any of the FTB pack is my ultimate point.
Except don't pick Agrarian Skies or Blood N' Bones (at least if popularity is your prime criteria); BnB is a very good pack but it's also very stuck in the hardcore niche, you will find dedicated loyal players but not a large number, if you get 10 together at once I will be very impressed.
AS is in a similar boat but the map it's self also adds extra complexity that only admins dedicated to providing the best AS experience could achieve, you end up doing a lot of generating islands for every new player that joins and I assume all this would be semi-manual.