Thinkamancer, for the communications capability, link possibilities, and spells to enhance the thinking of your own units. One of the entries said intuition is a quality that can be boosted, and Maggie cast that spell to reduce Parson's fatigue. I'd probably like my Thinkamancer to do more along those lines than what Maggie is doing right now.
Dirtamancy, for the upkeep-free units, incredible bonuses and healing to those units, and tunneling for retreats and surprise attacks.
Dollamancy, for the upkeep-free units, incredible bonuses and healing to those units, and awesome equipment. I'd probably like for every Warlord, and the units they're stacked with, to have access to some mean weaponry. Plus, raiment.
You can probably even link the Dirtamancer and Dollamancer together to get super-units, AND gem-studded raiment. Think about it: Elvis had those gem-studded outfits, and the Titans are Elvis impersonators. Pair your Dirtamancer with a Dollamancer, and you might get raiment that is truly Titanic.
Then there's the fourth one. I'm thinking Healomancy, Hat Magic, Dittomancy, or Signamancy, in that order.
It all goes back to my basic themes, in strategy situations: I like to strengthen and preserve my own forces as much as that's possible. To that end, in Erfworld I would like strong units, supported with healing and nice stuff.
So Healomancy is just the obvious fourth, for me: I can heal my golems, dolls, tchotchkes, and action figures, so why wouldn't I want to heal my other units? This is pretty much my style in every game; I like healing and defense bonuses to keep the units I already have alive. Basically, I like the idea of having an awesomely-powerful and well-equipped stack containing all of your casters and your Chief Warlord for insane bonuses and combat power. Consider: a Dollamancer and Dirtamancer giving bonuses to all the units they created, the Thinkamancer casting spells to boost everyone's insight and reduce fatigue (which we know is possible), and a Healomancer healing any injuries that might happen anyway. On top of that, the Healomancer linked with the Dollamancer can probably produce items for healing units. Maybe you can give all your Warlords and Casters something like a Ring of Regeneration? And didn't one of the updates mention Jetstone's Healomancer having a Wand of Cure Incapacitation? He must have made that himself, and THAT is some pretty nice stuff. Healing potions might be nice, too, but so far we've only seen a gin drink that he called a healing elixir.
Hat Magic is second on that list, and it goes back to that "nice stuff" and "free units" theme; we know that a Hat Magician can protect units from certain spell effects, and we know they can make snow golems, so they can probably make other units as well. Protect your units from Shockmancy and Foolamancy, and what will you have to fear? Plus, they can manage some awesome attack spells, giving them a way to do damage directly as an alternative to fighting. Not really my style, but I won't shrug it off if it's an option. After all, we've already seen what Dollamancy and Hat Magic can do together, even when they're not linked: they turned a scrub like Slately into a reasonably effective fighter. Imagine what a linked pair's equipment can do for an experienced warlord!
Dittomancy is third. You get free units, albeit temporarily, and again, I'd imagine you can get some pretty good stuff out of linking them. Maybe you can pair your Dittomancer with a Dirtamancer and get a permanent double of each unit the Dirtamancer produces? Maybe you can pair your Dittomancer with a Dollamancer to get equipment like the stuff Ace provides, only it's twice as powerful, or shoots twice as many shots? But, then, we don't know that Dittomancers can make stuff on their own, so it's hard to be very confident about this unit.
Finally, there's Signamancy. It's hard to say what Signamancy does, but from what I can tell, it must give the unit some manner of bonus. For example, people change their raiment to make dance-fighting easier (another reason for the Dollamancer I guess), and Slately asks for fearsome Signamancy before his big battle. That said, while I know that a Signamancer can do useful things out of combat, I'm not really sure what they do IN combat. I guess... giving the enemy BAD Signamancy, somehow? But that seems a bit weak. So, it's sort of the opposite of the problem I have with the Rhyme-o-Mancer and the Dittomancer.
Really, this brings another idea to mind:
Dollamancer, Signamancer, Rhyme-o-Mancer. Your Dollamancer and Signamancer give your units good raiment and Signamancy for dance-fighting, so they can own when your Rhyme-o-Mancer starts the dance-fight. I still like Thinkamancer/Dirtamancer/Dollamancer/Healomancer best, but this would fit my usual style somewhat, too.