I'll have to go with "THE DUEL" as well. Ewan and Hayden did train for countless hours and the end result was that the scene came off as though they were actually experienced jedi battling eachother, where as too many of the other duels come off as actors doing their best while being unusually slow and looking uncomfortable at it.
The battle between Qui gon, Obi, and Darth Maul was pretty awesome too, except that even it looked unnaturally slow at times, until you got Obi one-on-one with Maul, then the speed was absolutely fantastic, as both actors were great with the lightsaber. I only wish that the one-on-one between Obi and Maul bit had lasted longer, the energy of it was amazing.
Personally I think this has a lot to do with Ewan, even the stunt men commented that he picked up the lightsaber style faster than anyone they'd ever seen, including other stunt men. He does all the cool jumps and hops and twirls and spins his lightsaber with what seems like very little effort. One of the things that truly made him a fantastic Obi Wan is that he actually developed a real lightsaber fighting style.