Summon Evil Monster feat and Petitioners.

[Coidzor]

So the Summon Evil Monster feat allows one to summon the LE, NE, and CE forms of Petitioner.

What base creatures would actually be useful as something that could be summoned with Summon Monster II or in batches with SMIII or SMIV?

Bear in mind, it has 2 Outsider HD, a slam natural attack, and the ability scores and size of the base creature and that's pretty much it.

[Florian]

If you want to annoy your dm, spam Great Wyrm dragon petitioners.

[CasualViking]

If you think that lets you summon anything you want with the petitioner template, rather than the petitioner statblock already in the bestiary, I predict disappointment in your future.