After his fight with Itachi, Sasuke is captured by Tobi who proceeds to tell him about the history of Konoha, the Uchiha clan and Itachi. The only lie Tobi tells is that the 9 tails fox attack on Konoha was a natural disaster whereas we all know that Tobi caused it. Sasuke even calls Tobi out on his bull****:

Itachi told Sasuke the truth before their fight. I guess my question is why was Sasuke so quick to believe Tobi over Itachi?

Sasuke may have been confused about all the lying that Itachi had done, but Tobi still told Sasuke the truth about Itachi being an undercover agent for Konoha.