Frankly DL numbers could be deceptive, DW20 pack has Direwolf20's influence that will get it the DL's but that doesn't mean they will stick to playing it. I mean I didn't, I think I've DLed all the packs, some I've DL'ed multiple times.
The FTB team has NO reliable way of determining players actual time investment in each modpack, at least without using shady measures that would equate to spying on players. Which I hope think they don't do, hence why Jaded is mentioning DL numbers, as that is information they are entitled to and kind of need. Like a company knows how many units they sold within 99% accuracy but can't easily tell how much you like the product without asking or being told. High unit sales can equate to satisfaction but this depends of whether the item is worth returning if unsatisfied. This goes double for FTB modpacks as there's no returning of mod packs, I bet some people don't even delete packs they don't like.
If I was to put on my pundit cap I would actually wager that if the FTB RF standardization thing happens in 1.7 DW20 might see a slight rise in usage as some people can't do without their EU and MJ based staples. DW20 doesn't quite look like he's ready to forgo IC3 and the like just yet. Then again if he thinks the FTB teams idea was smart, you never know.
Right now though nobody can really tell you which packs are actually popular and in use, at least not enough to reliably base the decision you want to make. You will have to do your own research on FTB server listing sites and tally up the top 20 servers to see what they use.
Or you could do it the easy way and just pick the pack you like and stick with it. For all you know the most used pack might not be the one people want to play. The objective of most server admins tend to not align completely with the wants of players. An admin might pick the pack that causes the least technical headaches but isn't the pack that's most fun.
It also depends on the region your server will be based in, I can count the number of reliable Oceanic FTB servers on one hand I don't think there's a good Horizons server in Oceana so if players in that region where dieing to play that pack they can't so they play another pack.