RDM is the fastest to learn.

I dabbled in SMN in 3.x (main PLD from 2.0 to 3.2, then started a new character to main BLM in 3.x) and have only played the RDM to level 53 but I love the simplicity of the RDM job. It is the perfect caster dps to accompany whatever main job you already have. I now exclusively run the daily roulettes using my RDM because it's just so much more enjoyable to know that I can have a relaxed time yet still do my part properly.

If you're looking for a change of pace but don't intend to spend time analysing rotations and so on, RDM is definitely worth a look. After all, you can just do the job quest and you're ready to take it into the Lv 50 dungeons for a spin.