I almost hate to admit it, but I'd say Party Poker. Their software upgrade was long overdue. The gameplay has improved, especially in the sit and go tournaments. I also consider the fact that they dispatched two of their highest-profile players for cheating an accomplishment.
I love Pokerstars, but last August when they had absolute proof that Marcel Luske and Noah Boeken has engaged in a similar form of cheating they stumbled their way through the situation and ended up doing nothing about it. I guess the names were too big. To be fair, recently they have fallen into line and took prompt (and drastic) action against the same two players identified in the Party cheating episode.
I can't wait to see if there is a companion "worst poker site screw up in the last year" thread. I'd lead off with the above Pokerstars deal, but I have at least one other worthy candidate...