Well, I don't really dislike any factions in the Battletech universe and almost each and every one of them has a character that I like, so I'll name characters from the Free Worlds League and Federated Suns, the traditional enemies of my favourite Successor House (and I have to admit that I, actually, quite enjoy playing for and reading about both of them and consider myself a latent FWL\Regulan fan):
Jackson Davion and Titus Cameron-Johns.
Granted, the latter got much less screentime than the former, but I like how he started as a pacifistic prodigal son of the Cameron-Johns clan and turned into a ruthless nuke-tossing militaristic leader after a personal tragedy shattered his family life.
As for Jackson, I admire his loyalty to the Suns and his illustrious service that remained so no matter who was at helm on New Avalon.