lvl1; The Dire Badger. Not many options here, really.
lvl2; Ether Scarab. Damn useful for lower levels.
lvl3; Small Elemental. Earth, in particular, for earth glide scouting. Cranium Rat Swarm is also excellent.
lvl4; Salt Mephit is nice, I guess. Celestial Lion is cool too.
lvl5; Better Cranium Rat Swarm is great, once again. Also, Hound Archon is neat, and looks awesome as well.
lvl6; Janni. Chain Devil is great if you have chains prepared in advance.
lvl7; Arrow Demon is outrageous, and the inspiration for a character build of my username's likeness.
lvl8; Colossal Monstrous Centipede. With Rapid Summoning, two of these per turn are great meat shield. As usual, the Cranium Rat Swarm.
lvl9; Elemental Weirds are very multi-purpose. Additionally, taking the option fo summon multuple Centipedes from the previous list is still viable.