Actually, I thought it might've been taking the usual "torture O'Brien" theme a bit too far, but on the plus side, if you're not crazy about that episode, you can honestly just pretend it never happened.
As someone one pointed out after I mentioned about that, O'Brien got most of the torture plots in large part because he's the most centered, normal guy on the show. DS9 is the only Trek series were the number of alien characters outnumber the humans two to one form the principle cast alone, and between the three of those, one is a Bajoran god and the other is superhuman.
O'Brien already came in into the show with a family and his own history of war, so I'm not sure there was much to be done to change his character. And I'm actually okay with that, given the most of the cast does.
I would say my own least favorite character would be Jadzia Dax. It took me a while to figure out why I didn't care for her, but I think it was that she got the least amount of development that was about herself as Jadzia. Many of her stories before season four were all about Curzon or one of her other hosts, and many of the ones after season four were about being a Super Badass Klingon Liason--and we already had Worf covering that front. For this reason, I actually liked Ezri a bit more...because she's struggling to figure out about having this new identity that she never wanted or asked for and keeping her own personality as Ezri straight.
And as aside, that reveal about the true reason Curzon kicked Jadzia out the host program is some lazy-ass revisionist bullshit, and frankly a bit sexist on the part of Star Trek. Coming from the same show that did a lovely allegory on same-sex relationships, that sort of crap was waaaaaay beneath this writing team. But then, so it "Profit and Lace."