This time around, the best class/spec at a moment is just the one with the least amount of pvp template nerfs.
It was the sole reasion why frost dk dominated the pvp scence for a while even in arenas.
Most pvp is done in instances. Wpvp could be fun, but its restricted when it comes to rewards and honor gains.
In wpvp, i would go with a druid tank or dh or the ret. What counts here is not just survival, but mobility, too. Both rets and dks could use combat horse, but its restricted to only 1 hour, but if they use them and also those mounts in stormheim they are pretty tough to get away from.
My rogue is strong in wpvp too, especially with 2 survival legendaries and no pvp stat template to hurt his burst/dmg, however reduced slows to a flat 30% really hurts catching HK sometimes. I often think twice about attacking a dh, druid or monk, because they are so fast escaping.
i dunno why someone needs to play plate, but i would go: warrior for instanced pvp even solo ques and ret for wpvp. Dks are more hybrid creatures with tons of range abilities and even pets, they just happen to wear plate, other tha blood dk, they got pruned of most of their selfheals, it hardly recommend to play one solo, other than maybe an unholy dk in the backline using range abilities and playing like an affliction lock.
In short: You simply must like the playstyle of a spec and look at the pvp template. Class design is crap and won't help that much as in previous expansions, due to the pvp restrictions that could be made more visual ingame.