My least favourite so far would be Nemesis Inferno. It's actually the worst Inverted coaster I have been on. It's aestethically pleasing, has it's own cozy atmosphere, the queue helps build suspension and actually is okay themed. The smoke is cool as well. Ride-wise, it has a great view, and is good fun. The bit before the lift is cool, really makes you look forward to the rest of the ride. The first drop is good fun, looking at the walkway below you. The loop is okay as loops go, just so smooth and comfy. Then the Zero-G, the helix, the wingovers... smooth and good fun, you feel that you are tossed around and gently put down again, it's desorientating but not violent, and overall well into my top 10.
In other words, the worst Invert I've been on.