The best caster is Frost Mage. At high ratings, spriests Must pair with a mage or warrior - because a spriests job is support. Spriest + Mage, or Spriest + Warrior comps make up over 90% of spriest comps above 2400 this season. The other 10% consist of Spriest+Feral and Spriest+Lock (shadowplay, always destro or demo though now - because no one needs double support specs). Spriest is the best support DPS class right now. Affliction is an undervalued but competitive alternative Support DPS class, they have less team healing - but more team mobility (portals), CC (spammable Fear), damage reduction (curses), and defensive dispels (unbound will for themselves, imp for their healer).
With spriest healing nerfed (via pvp power), affliction is in the prime spot to overtake shadow as the best support spec. This is because if the only time you are vulnerable to losing a teammate is when your healer is CC'd - an imp-wielding aff lock can do more about keeping their healer in the fight than a spriest can. A spriest has mass dispel, which has a cast time, twice the cooldown, is targeted (can be moved out of, takes a second to target in addition to the following cast), and if the spriest is CC'd or interrupted - the dispel doesn't work.
Nobody CC's the imp, or kicks the imp (also because its instant, its not even possible to do so) - people do CC or sit interrupts on the spriest when they CC a healer and go for a kill though: that's not
possible against affliction. Mass dispel has a colossal mana cost, frequent use of it and our very expensive level 90 talents, and our healing - run a spriest OOM 2-3 minutes into any real arena game - not the case for affliction - their utility performs at 100% until the match ends.
When warlocks realize their best chance of success is to not DPS, but to play an affliction spec with the mindset that they are a support class - not a true DPS class - affliction's representation will skyrocket. It's not as fun as blowing all your cooldowns and blowing people up - that's your teammates job (ie. frost mage, warrior, feral) - your job is to roll curses on everyone, to call a fear target, to chain fear them into DR, and then to call the next target and repeat, to always run away at the first sign of taking damage, and to keep your healer dispelled the whole fight - 5.0+ Affliction is basically Shadow prior to s8.
You probably won't know your team scored a kill until your fear/curse don't land on that greyed out enemy unit frame - you won't feel like you contributed to the kill - but in reality - support classes are amazing: and locks that play like support rather than focusing on their DPS are wholly viable alternatives to shadow, with some real advantages and disadvantages to shadow. It's a good spec, don't knock it until you try it.
Also, regarding Afflictions damage being useless because its not burst, that's not true either. Yes - to score kills you have to have burst - but you don't have to
be the burst on your team. A frost mage or warrior or feral or hunter or rogue or ret or frost dk or windwalker or etc are all capable of killing someone who is at half health and doesn't have a defensive cooldown active by themselves (without your help), either if that opponent doesn't have a healer bombing heals (and possibly cooldowns) into them - and sometimes even if your teammate just scores a lucky crit or two. Your teammate is (hopefully) intelligent, they will always pick the low health target to put their burst into.
The advantage that pressure classes have always had (and will always need to have) is that they aren't cooldown dependent to put out their pressure, burst classes usually are cooldown dependent for their burst. So whenever your dots just happen to bring someone too low, that is precisely when your teammate will swap, and that is when your teammate will burst (because your teammate is intelligent). Pressure classes create windows of opportunity, they don't need to also have burst if their teammate can cover the burst requirements themselves: and every season there are always a fistful of classes capable of this. Having only pressure is rarely enough, and fortunately for Affliction - they also have a ton of utility - Shadow has similar or less utility, and similar or less pressure - but they have a touch of burst they can also contribute: but don't mistake afflictions lack of burst for lack of utility, or lack of pressure - it has reams of both, and both of those things matter a lot