When Efren first started making splashes on the scene there was quite a rivalry between him and Earl, with lots of finals matchups.
During the Camel Tour years there were some great rivalries. I remember the whole deal with Earl trash-talking at Busta, that was fairly entertaining.
The reason the women have clearer rivalries is because the top two or three women leave the rest in the dust, while with the men, anyone among the top 20 or 30 players in the world can win at any time.