My answer is based on experience as a 79 who mostly solo-queues and plays BGs only.
It very much depends on your playstyle. If you play with friends who will heal, or peel/stun for you, lock and spriest are super strong. If you play solo, those 2 classes are very easily shut down. The bracket is very bursty, so one interrupt or a stun will see your HP drop to almost 0 against good twinks. Though to be honest, any class with heal backup will do well.
For solo play, I consider mages to be the top tier ranged, due to very good dmg combined with kiting, blinks and defensives. The next closest ranged is a BM hunter, for much the same reasons, plus a very strong selfheal.
The strongest melee for solo play, assuming you don't want the rogue stealth and gameplay that goes with it, is enhancement shaman. You have one of the best bursts in the bracket, combined with very strong instant self heals. The drawback is you're quite kitable, so you'll have to learn to deal with that (Tailoring, FAPs, etc). No other melee can kill you, and only mages can give you trouble.
It's also somewhat important whether you will twink at 70 or 79. A mage doesn't get as much out of going to 79 vs staying at 70, for example. A shadow priest gets an execute, and I don't need to tell you what a gamechanger having an execute ability is vs not having it.
Best of luck twinking, and remember that playing a very strong class you don't actually enjoy that much is not ideal. Yeah, I like the 11k+ chaos bolts I can hit with berserk, but lock is kind of boring for me so I play the class rarely.