I think I can go ahead put Katamari down for a 10. It's one of the first games that come to mind when I think of the P2.
Katamari is of its own genre (you know, the roll stuff up in a ball genre) and completely unique. Originality aside, it is the epitome of what a good video game should be: unique gameplay that is easy to learn but challenging to master, simple yet relatable characters, intriguing (and absurd) plot with plenty of humor, and levels that increase in size and challenge from the diminutive to a near inconceivable scope. The sense of accomplishment is satisfying right from the get go and continues to reward the player to a ludicrous ending.
I've played all of the series and would probably go with We ❤ Katamari or Beautiful Katamari as the best of the bunch. But my favorite level is the "8 bit" 2D level at the end of Me & My Katamari on the PSP.