Ian McDiarmid is awesome as Palpatine. It is as if he is the only actor who takes his role seriously. Some memorable quotes:
"I will make it legal"
"Wipe them out. All of them."
"I love democracy. I love the republic."
"The dark side of the Force is a pathway to many abilities some consider to be unnatural"
Other than that, I would say the lightsaber battle at the end of The Phantom Menace is one of the best scenes, even though it was cut up to be interspersed with other battle scenes. I think that it was better than any of the other duels in the prequels. It was also influential in the fan community ... countless fan-films were made that had duels in them where at least one of the opponents wielded a dual-bladed lightsaber like Darth Maul's.