I know I only just started FFXIV a week ago - so my view in this specific game is worth nothing. However I've been playing a ton of MMO's (EQ, GW 1/2, DAoC, LineAge, WoW, Aion, Tera, Skyforge - just to name some of them).

The easiest to master and also best class (be it DPS, tank, healer or support) is always the one you like playing the most.
In my experience you're willing to go the extra mile in learning your class if you enjoy playing it.
If you play a "flavor of the week" class, then it seems more like a job to go for numbers and the joy of playing is gone for many players. This make their performance drop and they become tired of the class and/or game.