All of them, seriously, there is no best sub for jobs like thf,rdm,war, and a few others.
Certain jobs excel and reach their potienal with a certain sub in a certain situation with a wider variety of subs than other jobs.
Rdm as I said is one of them
Definites:
/whm
You'll definitely want /whm if status cures are a must. Examples include viruna on raptors, stona on cockatrices and a few other things, paralyenz(sp) on tigers and such, silena on tormas, poisona for crawlers if there is a pld tank especially. There are more examples but this gives you an idea.
However if there is a whm main in the party, you may not need /whm even if status cures are wanted. And sometimes even if there is a blm/whm, but if there is a blm/whm, but no whm/, you might want /whm so you can both split the duties.
/blm
If you're in a dungeon area this sub has obvious uses. Escape is wonderful incase blm dies or no other /blms or blms in the party. Conserve mp saves more mp than ballad or auto refresh if the rdm is a very active caster. But a conservative caster but be better off with another sub. Elemental seal is a good tool for helping emergency situation sleep stick.
/brd
My personal favorite. Gain the 4mp a tick /smn provides, AND do it for the other mages as well. Your base stats will should be fine too. You won't have the int of /blm or the mnd of /whm, but you'll have more int than /whm, and more mnd than /blm, so it's a happy medium in that way.
What makes the sub unusual is it provides a way to help the melee in a small but noticable way as well as an extra mp per tick to the whole backrow.
And by noticable, I mean throwing in valor II or something for that extra 30-40ish attk about. Valor II on brd with instrument when valor II can't grow anymore gives about 50 attk for the record. So maybe I'm giving /brd too much credit, but it can still give at least 20-25 extra attk at the worst.
This sub is however useless if there is a bard in the party already pretty much. AOE ballad is the main reason to sub this. And the negative is less max mp, but many rdms don't empty out much anyway. This sub is more for everyone else in the party than it really is for you. But in my opinion, rdm and brds are everyone else type jobs, there for supporting others. I guess it's how you view the classes and your own personal opinions.
/smn
Smn is my least favorite rdm sub of the big 4, but it does provide clear benefits, all of which go to waste on a tarutaru. Someone said this sub is better suited for galka, an that might be true I guess. But even galka rdms don't really run out of mp, I honestly don't see rdms run out of mp...almost ever.
Although blinkga is always cool for a party with SATA(WS) in it, or a party with bouncing hate.