ROGUE:
Gearing is expensive. Insanely high damage and strong CDs. Very good in pugs and in premades. You absolutely destroy priests and are good at returning flags. Lots of rogues but almost none of them are good. Good rogues prioritize objectives over KBs, communicate clearly, know where to be, when they should be there, and why. They also use stealth to be less predictable. For these reasons they are simple to pick up but have a high skill cap. You'll know a good rogue when you see one.
PRIEST:
Expensive to gear. Required class for premades and pugs, best CC ability in the game, best heals in the game, and dispel which is essential. You will be targeted a lot because you are an insanely high priority target. Communicate with your team and make sure you have peels. Priest is very versatile so you need to be smart about when you spend mana and what you spend it on. Pretty hard class to play well especially against players that want to shut you down.
MAGE:
Expensive to gear. Insanely strong in premades, weaker in pugs. Mage has pretty low damage output as a support class, but one skilled mage can manipulate an entire team. Speaking of which, very high skill cap. Most mages are garbage but the few that are good really stand out and get flamed for how annoying they are to deal with. If I was good enough to play mage at the top level I would, but I'm not. If you can get there, this class can be insanely rewarding.
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Priest looks really frustrating to play, you can do a lot but there's a ton of pressure on you and a lot of people want you dead/OOM. Hands down the last thing I'd want to play but maybe it looks fun to you.
Can't go wrong with rogue, and even if you're just decent at rogue you can still have a good impact. Definitely try to study the map and the pace of the game, macro vs. micro situations and push yourself to be better than a common scoreboard junkie.
Mage is super rewarding in premades but you have to know what you're doing. Not as fun in pugs unless your pugs are on the same page as you. I personally would probably pick this class if I wanted to really try and master something and feel rewarded for it.
if you just want to mess around in pugs I'd go with rogue or priest. In premades they're all good.