With how GEO is currently, its likely going to be /RDM. For the cures, haste, fast cast, and the MAB traits. The MAB traits are really important for being able to nuke, which from playing SCH makes it rather evident. /RDM gets the same amount of MAB as /BLM but offers more utility.

/WHM would likely be a close second only cause of the -na's and earlier haste.

If GEO had more than just mage armors, /NIN or /DNC might of been an option as with a B? club skill and WHM hammers the melee could of supplemented what else it is supposed to do while being up towards the front like most of its unique abilities want to put GEO.