LAST EDITED ON Feb-21-09 AT 01:04 PM (Pacific)Depends on what you like. Hitting level 6 or 7 is usually preferrable.
Create Life has the most durable level 7 summon, the Godzylli. Who does occasionally breathe fire but is great for stomping on things as a regular attack.
The Illusion level 7 summon is probably the least useful unless you summon in regular battles a lot. The Fantasmagora tosses out Death a lot if I remember correctly.
I've forgotten what Conjuration gave (was it a Fiero?), but I know it pops out a Myxlmynx at level 6. They're fun allies.