Lancer is probably the easiest, and it's also the least squishy of the DPS classes so you have a bit more margin for error.
The main difficulties in playing DPS come with off-GCD skills (Ninja), positional requirements (Monk) and proc-based mechanics that mean your rotation can vary dynamically (BLM). DRG has almost none of these - it has all of two positional skills, which you need to use about once every 20 seconds each, and it only has a few off-GCD skills that you just use whenever they're off cooldown - so in terms of things to handle with the rotation and stuff, it's pretty simple to learn.
