BLM and it isn't even close. It's basics are simple to understand and it has an incredible skill ceiling to boot. There isn't a single other job that plays like Black Mage in the game. It has consequences with reliable abilities for recovery and it's super satisfying to play. Every time I play BLM it feels like I have room to improve and that's fantastic. Paradox is also one of the best level 90 skills out of anything all the jobs have gotten.

Popularity means absolutely nothing except for sheer numbers and should not be used to determine whether something is good or not. Cid already covered this and it simply is not worth my time to beat a dead horse here.