When I really got the bug for live football it was early to mid 90's and for us, Villa were the main rival. At the schools in Bedworth it was City, Villa, Man Utd or Liverpool. Even then we all knew the Liverpool and Utd fans were a bunch of glory-boys, so it was us and Villa really.
Going away to Villa then was still a scary prospect, especially for a 15 year old, saw my first proper football fight right outside the ground after a match in 96 or 97 (I remember wondering how they could tell each other apart as nobody had colours on!!)
When we beat them in the cup it was quite dodgy outside, and the defeat that sent us down, there was a real feeling beforehand that it was going to be very heavy. It was the first time I heard of City fans going over with no intention of seeing the game, and tales of WBA and Blues firms looking for trouble as well. It was pretty scary after the game, and I guess that day drove the wedge in further.
Of course, the gap between the clubs, a bit of growing up, and probably a bit of apathy, means I don't really hate them nowadays, but I still fucking well hope they get hammered every week.
Never really hated Leicester, I do think it's a younger thing (I'm 34). Dropping down the divisions and coming across Blues and Wolves really made me dislike them, probably as much as Villa. Strange because growing up they weren't even on the radar, neither were Leicester really.
So I suppose I don't really "hate" anyone nowadays.
Leeds and Sheff United can get fucked though...