I was going to suggest priest. I had a fully twinked F2P 10 priest and it was amazing to play. 1 button. Then he got 7 days free game time so its technically a vet now.
https://worldofwarcraft.blizzard.com/en-us/character/us/alleria/gardivore
Build is very easy. Holy. Holy has much better SP damage scaling then the other two specs.
Because the only talent worth getting is Holy Nova.
Basically you can use SW
ain and spam holy nova. If you want to do single target dps, smite is actually better then mind blast once fully geared in Warforged WoD Epics.
Mastery is completely useless so you go for as much Int as possible prefer crit/haste/versa gear. If you get socketed epics, +3 int gems are available from the PVP vendor. Marks are harder to farm but you can get a few if you do enough garrison quests.
Sometimes I que for dungeons put tank on follow and just spam the holy nova button. Its funny to see them confused why everything is almost instantly dieing.
But if you have two computers you can run yourself through dungeons so easily with one button. It takes several minutes to OOM spamming, most dungeons are shorter than that, and basically if you are oom it just slows your cast rate by about 30%.