#17 said no one rivals his power even though someone who's been shown to be far more accurate than #17 has ever been says otherwise.
#17 says he's the most powerful being on the planet even though Piccolo demonstrated that he obviously wasn't. He only admitted that Piccolo was his equal towards the end of the fight.
#16 told #17 to leave because Cell was too powerful and what does #17 think? He's strong enough to handle Cell because he's the "strongest".
Cell nearly killed Piccolo with no effort after taking his Light Grenade (that #17 had to get away from) with no damage. What does #17 do? He still tries to fight Cell even though it's made even more clear that he has no chance against him.
My point? #17 is often full of shit and obviously has a tendency to overrate himself. The only thing to take from his statement is that he considers himself the strongest on the planet. By how much or whatever is all a matter of opinion.