Learning proper rotations, knowing the fight and having exceptional uptime is key for high dps numbers. Don't let others fool you, each DPS isn't all that hard to play well, certain jobs just tend to attract those less inclined to actually try to min/max, which gives an illusion of being more "difficult". If you possess good button mashing and aren't prone to panicking, you'll achieve high dps on all of them.

BLM is without question the ideal caster, at least until the expansion. It has access to self damage mitigation to reduce the need of movement, a gap closer of sorts to reduce movement time, and the ability to reduce magic dmg on a PT member. Assuming cross classes are lvled it also has access to STR down Virus and E4E. Move less cast more, this comes with learning fights to anticipate when/how you should handle mechanics for optimal uptime. That aside figuring out/reading up lil tips will increase DPS. (Not a BLM expert, so I'm not aware of all of them)