Recommend a good Summoner PrC?

[Mistwell]

I'd probably go the route of the Necromancer summoner. Use those new Summon Undead spells, and buff them with lots of the new Undead Buffing spells in books like Libris Mortis.

 

[farscapesg1]

If you are willing to use a custom prestige class, take a look at my signature. I don't know how it would fit into your campaign setting, but it worked for my DM. However, I never to to try it out in actual gameplay becuase our campaign crashed and burned during the Shackled City Adventure Path

 

[Twowolves]

It's old, but it's good, the Summoner PrC in the Relics and Rituals book. Just swap the progression of bonus feats with the bonus spells (so you actually get a spell to apply the feat to) and you are set.

 

[blargney the second]

Dreamsight Shifter Druid with racial substitution levels, a few levels of Moonspeaker, and the Ashbound feat. Rapid, spontaneous, massively augmented, very extended summoning. (Your summons get +4 Str, +4 Con, +4 to +12 untyped to a physical stat, +3 attack, and a bunch of other really cool goodies.)

And you're a druid to boot.