I am going to bet WAR, SCH, RDM and DRK for this one. WAR for its defensive and offensive boosts. DRK for it's ability to function as a glass cannon, even at subjob level. SCH for its spell boosts and some additional magic skills as back up. RDM for fast cast and additional buffs/enfeebles.

SAM I can see being useful for melee but might hinder things on the magical side. Then again, situations may permit it, depends on how SE's thought this out really.

However, I suspect WAR & RDM may end up being the 2 favourites. Well, it's a hunch anyway.

If it manages to tank well, WAR and NIN would be obvious choices.