As a boy growing up and being a passionate Newport RFC season ticket holder I hated Cardiff with a passion. And the fact they always beat us by like 2 points every time. Add that it was our biggest derby game every season, I really fucking hated them. This is back in the good old days of professional club rugby in Wales. Rivalries are almost non existent these days with the regions
Never really properly hated another team in sports though. I guess I really dislike the Miami Dolphins.