Protoss
Most: Immortals - the idea of updated dragoons actually supported by lore is just amazing.
Least: Mothership - they've messed with it too much and now it's the least exciting unit
Terran
Most: Viking - the mobility and flexibility it provides makes it the strongest unit out there. Terrans need something this flexible. Or the Battlecruiser, for obvious reasons.
Least: Not sure, either the Reaper or the Medivac DS
Zerg
Most: Queen - so happy it's finally a useful unit!
Least: Infestor, only because I haven't seen it in action yet and I'm not too sure it's going to add anything new.