There isn't a single best choice, there is a worst (like say SMN or something), but not just one best. It comes down to your opinion of what type of RNG you would want to be. The best choices would be sub WAR, THF, NIN, or SAM. Anything else wouldn't help much.
Subbing WAR makes it a little easier to solo by adding some defense and strength, rounding RNG out a little bit. You also do more damage then a RNG with the other subs, but won't hit as often. Main best thing about subbing THF is the extra gil that you make and items, making it much easier to support RNG, which is IMO the most expensive job. Then NIN gives you a huge accuracy plus, and you also get to use Blink. Plus, both NIN and THF sub's give you good AGI plus, which what every RNG should want. And SAM will give you a ton of TP really quick, and the Bow/Gun/XBow Weapon Skills are IMO among the most powerful in the game, so being able to use them twice as often is deadly.
So it all comes down to opinion. Would you rather blow everyone away with crazy damage as a WAR, or do a little less but hit more often as a NIN. Or would you rather make it a little easier to fund the job and sub THF. Or just go all out crazy and do a Weapon Skill every fight.
Personally I like WAR, which is hard at first. The other jobs have good AGI +'s while WAR doesn't, so its a little tougher at start. But once you get to lvl 30, it prettymuch doesn't matter anymore. Now at lvl 40 I think I only miss about 10-20% of the time.
Your choice :D
+ 12BLM, 10WHM, 2 RDM! lolmage
Merit 405~ lolLJ