I chose "Don't care", because it's the closest to "Neither".
Anyway, Shadow. Yes, Shadow. "Ew recolour".
Well, to be more specific, SA2 Shadow. Shadow from the other game can go to hell.
When in SA2, it was great. Finally an antagonist with similar abilities to Sonic, a worthy opponent. I was really pumped first seeing him, and playing him. Mysterious. I like that. It was the final realisation of Maria's final wish that got me liking him, and finally sacraficing himself for the sake of Earth and it's inhabits. I found him a rather deeper character than the others, always wanting to find out more about him. Too bad Sega started milking him, with no milk to milk.
After SA2, it just didn't work. But SA2 alone gets him as my favourite.