I am going to break this down one more time for you because I was at work on my first message. So this will be more in depth. I literally have 24 different level 20 twinks, and I highly recommend any of those three caster classes you mentioned.
Warlock. Firstly, mobility is trash. However, their damage out put in insane. You can single handily out dps any class in the bracket, if you have the right team competition and your team is willing to support you in the mid game. Soulfire and chaos bolts can one shot if you are near BiS on any target with global cool downs and mastery trinkets. This is a given. Undisputed truth. They have a nice tool kit. Infinite slow to make up the lack of mobility with curse of exhaustion. Health stones which stack with heal belt I believe which makes them harder to kill. They also have drain life which does insane healing intake. Great if you are good at fake casting, its nearly impossible for a melee class besides a hunter to take you out if you're spamming it. They have nether ward which is an insane game changer in arena or battlegrounds. You can reflect enemy spells. Whats more to love about hojing a paladin who trys to stun you. Also, they have unending resolve which Grants immunity to interrupt and silence effects plus gives you damage reduction on you by 20 percent which comes in handy if are running heavy on verse. Most destruction warlocks will not be running verse though, so either way this comes in handy. You also have fear, and your pet has succ pet has a slow. Your voidwalker can get targets out of stealth if you are crafty too.
Priest. They are the kings of healing in this bracket with the exception of Paladin being a near contender. They take more damage than paladins but are extremely hard to kill because most classes cant kick their holy and shadow spells sub-sequentially. Either you have to kick one or the other. They lack mobility but make up for it with insane team fighting contribution. Their bubbles are hard to break through when you are fully geared. They also have fear, schism which increases damage out put and can land big bursts with penance. Also they have shadowfiend. They have insane pvp talents to pick from. Double dispel into purification. Blaze of light which increases damage of smite and penance by 15 percent plus slows the target or increases the friendly targets speed by 40 percent when casting on them. Dark archangel is great for team fights and arenas when you can have burst synergy on your atonement targets. Strength and soul combos well with bubble for more damage mitigation up to 15 percent damage reduction when the bubble is continuously on you. They also have desperate prayer that increases hp by 25 percent instantly and heals you for that amount with only 1 and half minute cool down. Almost like a battlemaster trinket would.
Holy priest is a sleeper class right now. A few great priests like mindmaster play holy. They do incredible damage out put, and are extremely hard to kill one on one with desperate prayer and holy word serenity in the same tool kit. They also have a talent called miracle worker that allows you to have double serenity. Which is quite insane because its a double instant heal that brings you back to full hp two times over.
Shadow priest is my favorite of the three. You have dispersion which is great to adsorb large amount of bursts, and you can cast Psyfiend while you are in dispersion. You get desperate prayer as well, which makes shadow extremely tanky and very hard to kill one on one. Their tool kit is filthy. Especially tauren priests with warstomp and frag belt. Including fear, you would have three options to stop targets or increase your odds of living. Unfurling darkness talent allows you to cast vampire touch twice back to back for huge uptime on your dots on multiple targets or huge burst potential on one target if set up appropriately. Pysfiend and shadowfiend combo are actually dirty. Having both those on you at once is extremely annoying for any class. Pysfiend also slow the targets movement speed to help you with your lack of mobility. Devouring plague also comes in handy because while its an instant cast with instant damage it does damage over time and you reap the reward of 50 percent healing back to you as well. All in all its an S tier class in my opinion and those who are good at the class are hard to contest with and become extremely annoying to deal with.
Lastly the mage. Mechanically if you are good at this class it goes along way over the other two classes. If you know your rotations on frost and fire you can blow targets up. More specifically on frost, but fire mages do have potential. Sadly hardly any mages play fire, except for myself, But I have landed back to back pyros for over 1k a piece with globals going. Critical strike procs are the only way to enhance the full potential so you would need to max out your crit strike as high as possible, with the most versatility as possible as well. Its so satisfying to see a target get decimated as fire. Because I am not a frost God, here is migli a gladiator player in the 20s bracket doing a frost combo. (SEE DOWN BELOW FROST BURST) From what i gather they have huge potential with burst, are extremely handy in team fights and crowd control. They are hard to kill with versatility stacking and their frost shield absorbs quite a bit of damage. Arcane is also heavily played, but not like in BFA. They have the ability alter time which is just nasty at this level, a mass invisibility and blink which makes it incredibly hard to catch you. Anyone who is mechanically gifted, aka Wizkid, or other great mages have the ability to use this to their advantage. Sheep and frost nova help with your mobility too. They damage is obviously there. Either spamming arcane missiles or arcane blast it makes no difference. They both can top the charts.
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