Locks are strong enough to go with basically everything…
Lock /Shadow for the shadoweaving/curse of shadows double synergy
Lock/Mage or Lock/Rogue for the CC potential
Lock/Ele for the stupid burst
Warrior also tends to be a good partner, but ideally in a group of 3 so the warrior has heals/support… better really for the warlock than it is for the warrior, though the lock can counter hunters and mages for the warrior, and the warrior can peel off any melee on the lock/force any opposing caster to expend all their energy trying to keep the warrior off of themselves.
Other combos that are good
War/Ele/healing priest is imo the best 3 man team in the game. Warrior with windfury and ele burst + priest heals w/ ele offheals is pretty tough to beat.
beyond that, there’s no upper limit to the number of stackable mages where it becomes bad, and if anything a 10man mage wsg premade is unbeatable.
Warrior/Hpal is quite possibly the strongest 2 man team in the game.
Honestly… most every class/spec can produce some obscene things in the right gear and with the right support. Some synergize better than others (imo like rogues), and some are just really really strong in any and every situation (mages, locks)
Yeah, I’m going to call false on the resto shaman. They can shock, purge and drop totems instantly but aside from their Nature’s Swiftness CD, they can’t heal on the move which puts them lower than any other healer imo.