Your favorite unplayable character option?

[Seerow]

If you're really willing to sacrifice damage output, you can break past AC 80 as a Duelist at 20th level when fighting defensively. It's not good, sure, since everyone can just ignore you and your saves aren't necessarily great either. But yeah, Duelists don't have bad AC.

Agree with all this, but still agree with the op that duelist is essentially unplayable. The class does Grant basically +20 AC over twenty levels but the core concept has so little support it doesn't mean much. (Also by the time you're really seeing that payoff ac is often mostly irrelevant).