Best summoned leaders?

[Szumo]

Sleepers are nice if you have nature.

[jaif]

leaders as in any unit that can move around the map independently.

[Cor2]

Somebody has to say it, Seraph. so there.

I like all the demon lords and the elemental kings a queens but they all pale to the Seraph.

[thejeff]

Tartarians are the other main late game choice. Only some are commanders, but they're worth GoR.

Earlier in the game, Bane Lords are a common SC/thug choice.

In a non-SC vein, I'd also suggest anything that expands or increases the magic paths you've got available. Spectres have nice randoms. Lamia Queens can add some diversity. Faerie Queens let nature get air magic, and healing.
You can work your way up in death with Mound Fiends and Liches as long as you've got a D2 caster to start.

[KissBlade]

Tartarians obviously.

[sum1lost]

Mandahas!

[vfb]

GoRed Bog Beasts!