Although I find all DPS classes "easy" to play, I will agree with others here that some are harder to "master" than others. It depends on the fight however.

Assuming a target that moves around a lot, or requires you to change targets at a moment's notice.
1. Dragoon
2. Monk
3. Black Mage
4. Summoner
5. Bard

Assuming a primarily tank-n-spank target
1. Dragoon
2. Summoner
3. Monk
4. Black Mage
5. Bard