I know this is going to be opinion, so please explain what your choice is.

I have seen an on-going debate about what job a Rdm should be subbing. /nin is great for when the Rdm wants to go out and solo, but in groups there is an argument.

Some people say /whm is the way to go, while others say it should be /blm. Having not played the class I really do not have an educated opinion and was wondering what other people who have played the class thing. Please leave an explanation for your answer.

Thanks ahead for the info.

Unless you have a WHM in your party already, /WHM is the main thing you should be subbing while exping and as main support in tank PT for HNMs.

/BLM is mainly used for Elemental Seal to 100% land Slow II or Sleep II/Sleepga.

/NIN should only be used in lowman or, if you have the correct gearsets, to tank.

Source: 75(99 when it raised) RDM on retail 2006-2013 and 75 RDM on Nas.

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If you mean while EXP'ing, you should most likely be subbing WHM. If you get invited to a party that already has a WHM, then sub BLM.

If you show up to a party subbing NIN, you'll probably get kicked. If you mean end game, then it's entirely situational.

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It's entirely based on the situation as previously mentioned. Sometimes -na spells from /whm are what you need, sometimes you might be more of a nuker and want /blm. In reality you want multiple subs leveled and then go from there.

Practice makes perfect in terms of your playstyle.

From what I played, people didn't mind /BLM but some camps at higher level/with more progression require a /WHM sub. I'd say you could easily get to 75 with just the BLM sub; you wont be doing anything like Riverne Site A or Labyrinth of Onzozo, and might force your party to run a /whm for -na support; I didn't level WHM past lv9 until I got RDM to 75 but maybe the parties didn't care.

At end game you definitely want the /WHM sub leveled as you'll be supporting a lot; elemental seal is also used so /BLM is also useful. It depends on the content and what jobs are available.
A lot of Aht Urgan stuff can be done as /BLM from what I have seen.

Echoing what has been said already, /nin is more for low man or soloing but it's a useful sub to have leveled for most jobs all around. I have found /blu with Cocoon support to be fairly useful too if you're trying to skill up your Magic Skills/Combat Skills on even match enemies at 75 but are too lazy to have shihei in your inventory (worked well enough against Rarab's in Bibiki Bay to cap all my skills) but outside of that I wouldn't bother using it.

Very niche, but SMN sub is nice for healing as a RDM in Misareaux Coast merit groups due to the extra MP, Auto Refresh, and Rolling Thunder for your melees as well.