I'd recommend picking a class, not a spec in particular. I remember in Legion I made my druid with the sole intent of playing feral. Now I don't care much for feral and I've learnt that I don't care for druid at all really.
Pick a class that you'll like to play all specs with.
Now for my 2 cents.
Mage is PROBABLY the most boring class out of the ones you've listed just because of the fact that they are pure dps, by that logic, druid is most fun and priest is somewhere in the middle.
Thing with mage is, the majority of them get bored easily. The only damage viable spec is arcane. like 99% of arcanes damage output is one button. You can spec as frost and serve as a CC king, or you can spec fire for some exciting crit filled game play (but have more than lackluster dmg output.)
Priests are a bit better than mages because you can play healer, dps, or both. Holy is great for pure healing output, it has some huge healing burst. Holy also does the most single target burst damage of the 3 specs. Discipline is a fun, easy to play dps/healing hybrid spec, this is the most common priest spec. Disc healing is normally proportionate to damage. More often than not my dmg and healing are the same while playing disc. Essentially you just spread dots on the enemies creating pressure for the healers do deal with, then just bubble those that drop in health. Atonement does most of the work and the only time you need to turret heals is when someone is taking big damage. Shadow is pretty mobile @ 20. Run shadowy insight, and SWP the whole team. Mind flay is a serious slow that can give any EFC a bad time. You don't do insane damage but your off heals / bubbles make for a great support spec. Void Eruption is also probably the hardest hitting AoE spell in the bracket.
Druids are probably the go to if you want variety in your game play. They can be tank, dps, and healer. Guardian isn't really viable in bgs atm, but its rly good in dungeons, but if you want to do some exploration tank specs are ideal because mobs can't daze you off your mounts in tank spec. Restoration is a top tier healer, basically impossible to kill unless you have like 3 or 4 twinks attacking you. Best mobility in the bracket, and high tier damage output. Basically godmode. Restoration druid is also top tier FC. The only thing that can compete with an rdruid FC is rshaman, even then, druid is still better. Boomkin has less survivability but can still be hard to kill, Their damage output is ridiculous, S tier rots / burst damage with the double starsurges. Druids got feral too which makes for a total of 4 specs to choose from. Feral can serve as great team pressure with the Lunar Inspiration talent, spreading dots and generally just harassing the enemy team. OR, if you decide to run bite spec, you can do some serious burst damage with the right setups, coordinating fero bites with other twink's is sure to one shot most twinks if not take them down to like <30% hp. You'll be able to stomp lvlers with ease.
I have a bit of bias, I say go w/ priest but that's just because that's what I have the most fun with. Objectively speaking, you should roll druid because it offers the most variety which means the most potential for fun. Arcane mage IS fun for about 10 minutes then you'll want to log.
At the end of the day, I highly doubt you'll be disappointed with rolling a priest or a druid.
sry if some of my grammar makes no sense the more I type the more my brain melts and the less I care about revising my messages x)