Obi-Wan Kenobi: You were the chosen one! It was said that you would destroy the Sith, not join them! Bring balance to the Force, not leave it in darkness!
Anakin Skywalker: I hate you!
Obi-Wan Kenobi: You were my brother, Anakin. I loved you.
From the movie Star Wars: Episode III – Revenge of the Sith (2005)
This quote come at the end of the epic confrontation between Obi-Wan and Anakin. Obi-Wan believed that Anakin was the Chosen One as his master Qui-Gonn told him. In this speech Obi-Wan expresses his emotions as he realizes what has become of his friend. The Chosen One is an old Jedi prophecy and at this point in timeline it was believed to be Anakin. He does fullfill the prophecy in the end, but not in a way Obi-Wan expected.