Zamalek-Al-Ahly in Egypt. The rivalry's so bad and intense that the following are done whenever they are playing:
1) The 2 matches in the League and any other time they meet are played in a neutral venue, Cairo National Stadium.
2) A foreign referee is always brought in to officiate. When Hugh Dallas blew the match, Zamalek fans wanted to kill him. Why? He gave Al-Ahly a deserved penalty.
3) No player has ever gone from one team to the other. Not even by way of another club. Hasn't happened, don't think it will happen. His own family would stab him...