My answer varies depending on whether we're talking face-to-face or online play. Amongst my friends who get together and play, Fellowship and Towers Standard are favorites by far (this is in large part due to a house rule/gentlemen's agreement we have that bars x-listed cards from play in all games - idea for a new format anyone? ;-)
As for online, I've been liking Towers lately. Although I have a strong, ahem ...dislike... for
Legolas DH, at least I stand a chance at cracking him when
Merry Friend to Sam isn't in the picture. Since I build my online decks based on my real-life decks, Towers and I get along great.
Though the combos in open/exp can be impressive, neither myself nor my circle of friends really bought any sets beyond the movie blocks. So, any format that has a ring-bearer other than Frodo and a non-linear site path (I'm a sucker for movie themes) finds me a bit out of my playing depth. That said, I'm always glad to see a vibrant playing community whether I play a given format or not :-)