I personally like a lot of the O'Brien episodes; particularly "Hard Time". Some of Colm Meaney's best acting is in that episode. What bugs me, though, is that he's completely fine in the next episode. He never seems to suffer any ill effects from all the torture he gets put through. *sigh* Star Trek and continuity have never been the best of friends.
Although Quark is one of my favorite characters, I can imagine a lot of viewers disliked him. He IS greedy, cunning, and manipulative, after all. And let's face it, Ferengi characters aren't likely to win any Trek beauty contests. :P