Black mage is easiest because its rotation is simple to learn and somewhat intuitive once you know what all the spells do and are used for. Black mage is hard because you have very little ability to do damage while on the move, and im sure one of the things that caught your eye watching those high level boss battles was how much moving around you have to do sometimes. Black mage does top tier damage when in rhythm, but that drops when moving(or dead because you didnt move). Red is a beginner friendly class like the other guy said. Bard can be a little micro-managey but its not too bad. Managing songs is always have one playing and know which order is ideal for your party. Beyond that its a class of proc, dot, and buff management.