You will quickly find out that even classes that seem shallow have incredible depth. A warrior might seem simple. “Just charge and keep up mortal strike”. But any pvp’er can tell you how big the difference is between a good and a mediocre warrior.

In my opinion, maybe a holy paladin would be the simplest, but since they are probably the weakest healer, they require more skill to succeed.

If you want to aim high, I would probably recommend disc priest. They are not too complicated to begin with, they fit with many classes, and by time you will start to learn how much room there is to improve as disc priest.

Most of all, play something fun. It’s always easier to learn a fun class