Depends on what your goal is. If you just want to do casual content, pick whichever is most comfy, they all work well. If you want to do savage, they all still work well, but sam will have the highest difficulty getting into groups. mnk is the most desirable out of the 3 for savage groups, and currently outdamages sam. Blm is pretty fun in savage imo, and carries its weight well if played right, but there's a bigger gap between average blm and good blm, so until you better master it you'll have difficulties.

if you prefer melee, go mnk since you can just use the same gear for sam anyway if you want to mix it up later. if you want to be ranged, go blm and get adjusted to its playstyle well. Most dps jobs are more active on the present, but blm is more focused on what it needs to do in 10 seconds than present in harder fights. It takes getting used to.