Any job that you can get that can disrupt any type of casting will be a good partner.
these are just my preferences, but for each spec you need a certain partner:
BM spec, i'd aim for warrior, pally, druid, rogue, shadow priest or warlock. you can effectively take one player down yourself while the partner distracts the other.
Mark spec, I'd go for warrior, druid, pally or shaman. This is more focused on you doing alot of damage in a short amount of time and having to concentrate on someone else. if you are forced into melee you can rely on your partner for a random heal.
SV spec, I'd go for a healer or a hybrid healer. With SV spec, you have alot of ways to CC a player and you can charge in focus more on the other one. SV also tend to have a good bit of hp, and better ways to annoy and get the attention of someone.
Once again these are my thoughts. As long as you have fun, you should win a fair amount of games.