Good summon nature ally creature(pathfinder)

[about50heavies]

what are some good creatures to summon for each level of summon nature's ally in pathfinder.

[Psyren]

Scroll to the end of this guide (http://www.d20pfsrd.com/extras/community-creations/treatmonks-lab/druid-handbook-part-3) by Treantmonk, he goes pretty in-depth on each level.