Whether it's the owners that have ruined it for you, the design of it, the make and model... What really pisses you off?
For me personally, Fiesta ST150s. I've not come across one example that doesn't have a stupidly loud exhaust fitted and isn't driven by some knob who always wants a race.
I also loathe the A class Mercedes, has to be the most finaced car in the UK surely? And seems to be driven exclusively by the type of man that likes driving fast Infront of school girls.
The new Ford Puma is starting to get that way for me. They seem to be driven my numpties. Either driven by people that drive it around like a go kart and sit on your bumper in 30 zones, or by old people that can't park. It's also tremendously ugly in the rear view.