First i want to say, after lvl 20 without a subjob, forget parties. Its mandatory.

First of all, WHM is the preffered subjob. For MANY reasons.

You get status cures
Get Cure spells faster as opposed to the RDM, example is CureII, at lvl 22 you get it as BLM while for /RDM in the BLM, its lvl 28, and alot of spells over lap. You also miss out on Divine Seal, and Regen and other stuff. BAsically you cant expect the WHM and RDM to always keep things ok, you never know how the /WHM willl save everyone and keep monsters comming instead of not being able to do anything. /WHM is the recommended sub and the one i would prefer. First thing i would look for BLM is BLM/WHM because they have more to offer.

/SMN is only useful at lvl 50 with Auto Refresh, but to me that isnt a means to forgo anything the WHM sub has to offer. So the only thing left is MP boost. But thats not useful when the MP has nothing to be spent on. Sure you can nuke more, but you prolly disrupt the PLD/WAR/NIN on holding hate, you rarely use up all your MP in a battle, and if you are at higher lvls, something is wrong there.

/RDM is basically anything a WHM can give and less. Fast Cast is NOT the 2hr, that is Chainspell. Fast Cast makes your casting less for each spell(passive trait). But theres no need for even that because more nuke=more hate, and you need to ration and make sure hate is minimal to you. Magic burst? i think that without fast cast you can know how your cast times are and fast cast actualy SCREWS it up more.

So /WHM is the recommended sub and /SMN being second....but a FAR FAR second. I prefer /WHM over any other sub. And i am sure the majority of the people prefer it too. /WHM on alot of mage type jobs cept the WHM itself is just too freaking good.