Gunbreaker and Reaper have to be neck and neck for the worst jobs with Black Mage right behind them.

Gunbreaker is straight up ass. Yes, applying the Tank spell can give you some survivability, but the amount of effort you need to play the class versus the amount of damage you actually put out is ludicrous. Like seriously, the damage you put out is abysmal. What makes matters worse, the entire job is tuned to DO DAMAGE. The only utility you have is a stun AFTER your ultimate ends. Warrior gets a group stun every 15 seconds and craps out damage during Inner Release. I know which one I'd rather play.

Reaper is also in the same boat. I just played a match where a Reaper was beating on me in the back line for like 10 seconds and I just completely ignored him. 0 damage class. Fear is a hit or miss. Unavoidable CC is good, but it can also screw your team over by forcibly moving people out of danger. Plus, with how busted other jobs are like Dragoon and Red Mage, there's no reason to ever play this class.

Black Mage... ugh. What a chore to play, and your entire strategy revolves around the best way to pray the other team doesn't focus you. I get that some people are able to go Rain Man with the debuff tracking, but in a game mode where AOE and/or burst is king, it just doesn't cut it. Did I mention it was a chore to play? Like seriously, it's gotta be the most unfun gameplay I've had in an MMO period.