Hm...so it's more 'concepts' in a sense. Besides that, it'd be unheard of to summon a divine spirit as a Servant. Demigods, sure--anything higher than that is far beyond the scale of the ritual.

[laughs harder in grand servants]

Gilgamesh, for example. Probably the strongest Servant one could summon, both by virtue of being part divine and because his is the oldest legend in human history. The older and more well-known a hero is, the stronger their potential--so he's close to the upper limit if not the limit itself.

[Beat.]

Kind of a prick, too.