To kill Itachi. The only ones in Konoha, during that period, who were atleast as strong as Orochimaru, were Jiraiy&Tsunade, and he desperately wanted to kill Itachi, and he saw how massively outclassed he was, during the Tsunade-Retrieval Arc. read more
Orochimaru, had already given him a small sample of his power(Cursed Mark Of Heaven), which basically was a power-incentive to Sasuke, to show that he could become much much stronger if he defects from Konoha and goes to Oto. read more
The main reason why Orochimaru wanted Sasuke rather than Naruto, was Itachi Uchiha. Sasuke Uchiha, was developing into the prodigy that his elder brother Itachi was. Orochimaru had experienced Itachi's power first hand. read more
Secondly, it turned out that Sasuke wanted to kill two birds with one stone: To get stronger, and to get Orochimaru out of his way so that he wouldnt be a problem in the future (not to meant that Orochimaru is a bad guy with bad ideals). read more