Top are: Bloodrager, Paladin, Ranger because they can cast spells and, by that, are more versatile without losing power.
Barb is the most powerful you can get without spells.
True, I forgot about bucklers being unable to be used as weapons. Still though, a few levels into paladin can help the Swashbuckler a lot.
You're also making the assumption that I'm talking the standard two-hander paladin. Paladins, thanks to their Smite Evil AC bonus, along with a few feats, can make decent turtles with a shield, even a buckler. That said, a Dex-based Paladin who takes a few levels in Swashbuckler can be very frightening.
