For DPS, I'd have to agree with Tristana. She hasn't gotten a change in a long time, which is generally a good sign.
For tanks, I really like where Blitzcrank is at right now. He used to be OP, but riot really hit the nerf-nail right on the head. He's still really useful but doesn't have anyone complaining, he's probably going to stay put for a long time.
For casters, Annie is probably your best bet, other than some bug fixes she hasn't gotten a change since they increased her AP ratios some months ago. Ryze has obviously been through a major overhaul and needs some fine tuning still, veigar is kinda bottom-teir and is more likely to recieve a buff, Anivia might give Annie a run for her money but has been changed more recently and might not be finished.
As for support, well, support is kinda effed right now. Exec calling used to hurt them really badly but since it got changed, they might make a reappearance. Heal-stacking used to be a problem and might become so again. Either way, anyone in the support class might be indirectly nerfed/buffed by item changes so its kinda hard to call any of them balanced.