I figure that this has already been the discussion of many previous blogs but i figured that i'd throw my two cents in here. THey say when you are about to die, your entire life flashes before your eyes. here is what i think Anakin Skywalker/Darth Vader thought and seen right before he died.

I betrayed the Jedi order once, and it cost me my life, i did not make the same mistake again. I failed you, mom, i didnt become the Jedi we hoped i would be but i have now done my part to restore the Order.

I believe he seen visions of his life such as the battle of Naboo, the Clone Wars, his days in the Temple, and his turn to the Dark Side. I also think that if this were to be a movie, it would have some sad, depressing music to it like the music playing when he made the decision to help the emperor in the first place. And that he saw visions of Obi Wan, his "friend" Palpatine, members of the Jedi Order, Yoda, Jar Jar Binks, his friends on Tatooine, and his beloved Padme.