Whm/???

[Ninmin]

EDIT: *DELETED* (this is now deleted please move,lock,and or delete thank you)

[Hantz]

Oh, it's in the stickies....

[Jugil]

/BLM is good for several reasons:
1.) it gives a nice MP boost (BLM = 2nd highest MP pool?).
2.) both WHM and BLM have enhancing/enfeebling skill, so you'll have access to a few more enfeebles (Poison, Blind, etc.) and buffs.
3.) You will get access to spells like Escape (Can really help your party in a rough situation) and Warp (good for getting home).

/SMN is also good for the following reasons:
1.) an even bigger MP boost vs. /BLM as SMN has the highest MP pool.
2.) at 50 WHM, your SMN sub will be at 25 (assuming you've leveled it), which means your WHM will have auto-refresh (+1 MP per tick).
3.) you will have access to SMN Blood Packs (though healing/buffing BP's are the only viable ones?).

/RDM is also viable for the following reasons:
1.) you will still get an MP boost, tho not as substantial as /BLM or /SMN
2.) dispel, dispel, dispel (really good if you're fighting mobs with many buffs like crabs).
3.) you will have access to additional enfeebling spells, as with /BLM. plus, RDM have the best enfeebling skill in the game, so you will get a boost to that skill.
4.) you get Fast Cast, which will reduce casting and recast time for spells.

There are some situational subjobs, such as /THF for farming and /NIN or /BST for soloing.

[Daver]

If you are just starting this game for the first time, I recommend BLM sub. You can't unlock SMN until level 30. With RDM you will have to buy 2 sets of armor and weapons as you level up, which can be pricey for someone starting out with nothing (especially since you need to spend your precious gil on spells). A BLM can share many of the same gear as your WHM and would be the easier route. You want to get BLM to 17 anyway (for Warp) no matter what your long-term sub would be. Once you have WHM at 30 and BLM at 17, you will have a better feel for the game and can try out different subjobs as well as main jobs.