for PvE it depends on if u want like a fun spec, one shot? fast movement speed? high single target burst? sustained dps?
i can imagine hunter being pretty good with hybrid mail/leather gear, but i havent seen any to confirm it.
Druid and Shaman are definitely viable for good dps probably being the best picks for sustained dps.
Shadow priest has probably the highest sustained dps for cloth casters, and mage has probably the highest burst in game(even if its not the best its still really good).
Havoc DH doesnt do a ton of damage as far as i know, but you can get over 100% movement speed with it so it should be a lot of fun potential compared to other classes.
i dont know how viable plate users are for dps, but all tank builds are viable as far as i know.
I strongly recommend NOT going caster for pvp, i dont know if melee has the same problem with missing 80% of the time, so if you're going to mostly pvp on your 101, then a Blood dk, enhancement shaman or a hunter is your probably your best bet.
Although it would be pretty hilarious to crit 5m on a 110 randomly in world pvp with glacial spike, its unprobable and its gonna take probably 20-30 tries before you actually kill someone.
All in all if you want a failsafe twink, make a blood dk or a enhancement shaman or a balance druid, you can't go wrong with those. but i can recommend trying to make your own kind of build trying to find out some new combo that is fun for you
I'm personnally gonna try out haste heavy build with arcane in 7.1.5 since theyre reworking some of the spells.