Hmm. Tough one here. I think I might give this to Terra. I wish I knew which of the two is the better Summoner because that might change things, but if they're equal or if Terra's just slightly worse in that department she'll still have Rydia beat in terms of magic usage since she knows Black, White, and Time magic. Plus, she has her Esper form too.

Ryu
Bayonetta
Dante
Kratos