I see many folks mentioning the P88 in regard to Walther. I would agree that it was their best 9mm of all time.
However, to be more specific, I'd say the rare SAO variants are the best of the best, such as the P88 Champion or Competition. They are a little better fit and have phenomenal triggers.
Below is my P88 Competition on the left with my P88 Compact and P88.