Well, here is my 2gils
As a Hume, I love my SMN sub, max MP boost, more INT, and refresh when I hit lvl50 BLM. (Best of all, as a BLM, I hate curing....lol)
If you are playing taru, then IMO RDM is best. More hp, some INT, and fast cast. Taru got good enough MP pool already.
And also IMO, I don't really like subbing WHM. Yes, you can heal during battle, but blasting and healing together....won't you just become a hate generator. Couple of lvl II spell, and Cure II or Cure III. But then again, if you are a master at hate control, subbing WHM will get the group most.
In conclusion, as a Hume BLM that doesn't like curing, I choose SMN over WHM or RDM.
exdream
BLM 35
SMN 20
RDM 16
WHM 5
Carbuncle