Stick with Nin. It's complex without being punitive, especially with the addition of Armor Crush post-50.

Dragoon is equally as complex as Nin at 60, but really suffers during disconnects from the boss. As an i180 DRG with my Hive Weapon I've come to almost loath the class despite the kind of dps it can pump out (Best in the game in the right hands).

Monk is probably the simplest of the 3 despite it's numerous positional requirements. At first they seem quite daunting but ultimately it boils down to Flank-Flank-Back, Back-Back-Flank while watching your Demolish/Touch of Death DoT and maintaining Greased Lightning. Post 50 you'll need to manage Chakra during your downtime, and get familiar with breaks in a fight so you can maximize Tornado Kick. Monk can be pretty frustrating at times as well since most scripted fights will make you drop Greased Lightning stacks and tank your dps for a bit.