If you haven't done any trading over at maxminis.com yet, I would *highly* recommend giving it a try. I just went through a glut of trading last week/this weekend, and now I'm sooooo close to completing most of the sets and/or getting all the figures I need. Granted, I was lucky that I had a few choice rares from older sets (Drow Cleric of Lolth, Drizizizizizizt, etc.) to trade that helped. If you have any duplicate rares, or even some good uncommon/commons, you should be able to find someone to trade with.
As far as the topic goes, I've been pretty happy with all the sets. Giants of Legend had some really terrific figures, but that damn Huge rarity distribution really turned me off. I bought three boosters before I walked away from it completely. Luckily, my mother bought us a case for Christmas - she rules.

I was happier with Aberrations than most (except for those blasted Celestial Brown Bears that showed up in *every* damn booster I opened!), primarily because of the Bladebearer Hobgoblin - Best. Sculpt, Ever., IMHO. If I had to choose, though, I liked Deathknell a lot (my wife is obsessed with the Giant Frogs), and Angelfire was really great, especially as far as commons go (I bought 12 boosters individually, and got away with only two Wrackspawns)

Oh, the day I bought two boosters and pulled an Ogre Mage and an Efreeti will long be cherished...
If you held a gun to my head, I'd probably have to say Angelfire is my favorite. Underdark is looking pretty good so far, despite its FR-heaviness (I know that's a draw for many, but not in my case). At least that should give me plenty of trade bait to get some of the other figures I'm missing!