The Reckoning. Apart from being beautifully shot and probably the closest interpretation of the medieval era, I'd say it's Paul's best performance. In my opinion it's probably the most underrated movie out there.
I amor all his performances - Paul is great even if the film isn't! My favs if forced to pick are A Knight's Tale and The Reckoning and ...... I could go on and on. Personally I think Paul is the best British actor around at the moment and so very underrated.