If by versatile you mean used by most number of main jobs, then definitely NIN. SMN, BLM, DRG, MNK, SAM, and PUP seems to have very few practical uses for NIN as a sub. Every other job (except maybe COR, I'm not sure about COR) uses NIN for some purpose. I'm of the opinion that /NIN is highly overrated in most situations. It's only practical use is shadows as dual-wield isn't anywhere near as great as everyone seems to make it out to be.

I'm leveling NIN now mainly because RDM/NIN past 74 is insane. Utsusemi: Ni, Stoneskin, Phalanx, Protect/Shell, en spells, haste, and refresh is just godly and makes for one of the best solo'ing job combinations in the game. I just want to have NIN ready for then. It's also a common sub for BLU, which I plan to level in the future.