Direwolf20, Horizons and Monster just feel so similar. Their differences are almost nitpicky. I wish there was a clearer sense of what makes each pack stand out from the other, because I simply can't tell at first glance. Or second glance for that matter. Obviously Dire's is basically just a tailoring for his LP. Same with the eventual Mindcrack iteration. But pre-1.6, the non-LPer packs felt more distinct to me. Maybe it's just because it's still early on in their development and there are plans to include certain mods to help the packs stand out, but the moment they're just a bit too similar.
MF2 is the most unique; I like how it centers itself around TiC and how focused it is in general. Feels like a slightly more vanilla TerrafirmaCraft - and that's a good thing. I haven't tried Magic World and Tech World, since I'm too indecisive to ever choose just one
And of course FTBlite is meant for slow computers, so for certain people that's the best pack of all.
That's the closest I can come to ranking them. Maybe once Monster and Horizons are in release mode I'll have a better feeling for which I prefer.