My all time favourite has to be the Mustafar scene.
Obi-wan: You have allowed this dark lord to twist your mind untill now, now you are the very thing you swore to destroy.
Anakin: Dont lecture me Obi-wan i see through the lies of the Jedi, i do not fear the dark side as you do, i have bought peace and security to my new empire!
Obi-wan: Your new empire? my alliegence is to the republic Anakin, to democracy!
Anakin: If you are not with me, then you are my enemy.
Obi-wan: Only a sith deals in absolutes.
Anakin: Dont make me destroy you.
Obi-wan: I will do what i must (Lightsabre ignites)
Anakin: You will try (Lightsabre ignites and parries Obi-wans blow) My old friend
Battle begins