Shaq tends to exaggerate when he says like this. When he joined the Suns, he said Amare was the best big man in the league. When he was with the Lakers, Phil Jackson was the greatest coach in the history of the NBA. He probably actually believes what he says at the time to a certain degree, but you have to believe he realizes that he's exaggerating. I believe Shaq believes Kobe is the best player, but not that he's that much better than everyone else. He's just exaggerating, and part of it is to do a little damage control about him being an ass.
As for whether I believe what Shaq says, of course not. LeBron is probably even better than Kobe at this point. Kobe is the best combination of skill, athleticism, and mental approach in the game. But, I think LeBron has now passed him as the better player. Fine line, but I'd take LeBron over Kobe now even though I like Kobe more than LeBron.