For DPS, I think DNC is the easiest as its resource management and burst window is really simple compared to basically every other DPS and making mistakes is rather minor.

WHM is the easiest healer as the vast majority of its tools can be used reactively as damage happens, whereas SCH and AST both require some degree of forethought to get the most out of their kits.

WAR is the easiest tank as they have only one resource to manage and an extremely small number of buttons compared to the other tanks. They also have the distinction of actually becoming easier to play at higher levels as Inner Release removing the beast gauge cost from actions makes resource management even less important for them, whereas prior to it you'd want to be pooling both gauge and Infuriate uses to get off as many Fell Cleaves during a Berserk window as possible.