I'm with KanisX on the old Buggy/Wartrakk models. They're only good as spare parts. As for other Ork models, I can't stand the Tankhammer model for the Tankbustas, and something about the SAG Mek is a bit offputting, maybe it's just the head which bothers me, which can easily be replaced.
I don't have much love for the Venerable Dreadnought, although the FW ones look great, and I don't like the new Ironclad either. Wraithguard need some newer, more dynamic models. I love the guys but they're so static! Warp Spiders also have some pretty boring models. I can't stand just about anything from Witch Hunters, particularly the SoB sculpts which tend to suffer from the same problems I just mentioned. Necrons are also generally pretty poor, and not due to a lack of interest on my part - the artwork in the codex is downright chilling, and I wish even the basic Warrior model could live up to that. Wraiths, Flayed Ones, and Immortals are all good sculpts, but they're also all metal. I won't bother with Dark Eldar - just about the only mildly acceptable models in their army are Incubi and Raiders/Ravagers.