I believe I can either placate or disable Robot with this statement:

Johnny-5 cannot be the best robot because you, Robot K1, are the best robot; you are capable of discussing the merits of various robots and thus demonstrating superior logic and reasoning.

Either this will make it happy, or it will begin to see the paradox included within the statement. If he is the best robot, his conjecture that Johnny-5 is the best robot must either be true, making him inferior, or it will be false, making him incorrect and thus inferior. If he is inferior, he is fallible and thus can make mistakes, therefore he is not culpable for his earlier incorrect statement, making him once again the superior machine.

Let's see how it goes. If he ends up sayings "Does not compute" and spraying sparks, I'll pay for his funeral.