I actually had a savegame for every possible combination of campain playthroughs, because it used to be impossible to edit your multiplayer spellbook - you had to play the game to get the spell combinations.
That said, this used to be my selection:
Level 1:
Stratos
(frostwolwes are great to micro with, and the brainiac's knockdown is amazing, as is the short recharge time on the lightning spell)
Level 2:
Persephone (healing bug - enough said)
Level 3:
Pyro
This is the last level of completely free choice. Creature-wise this level is redundant for me, since I will take a brawler next level anyway (Storm giants <3) so I go with the firefists since they at least are adorable, and rings of fire is a great spell.
Level 4:
Stratos, no contest. Storm giants are amazing, and chain lightning is possibly even better. Second choice here would be Persephone due to the awesome rainbos wpell, but her followups are weak.
Level 5:
James
Although the Icarus is kind of sucky, Halo of Earth makes up for it. Once spellbook editing became available, I used Charnel in this slot, and sometimes Persephone for the entertaining but kind of useless rain of frogs. The bombers of Persephone and Stratos have a very nice web ability that can turn battles if you're good at micro.
Level 6:
James
BOULDERDASH!
Level 7:
James
No choice in the campaign here, otherwise I'd go with stratos.
Level 8:
Charnel
Largely personal preference here - and in the campaign here it's a choice between James and Stratos, and the final creature for James doesnt fly so I didn't want to take it
Level 9:
Charnel
No choice, and the Hellmouth is a great creature anyway.