"Best" is subjective. The PS3 was a paradox: brilliant exclusives and media features, hampered by launch price, complex architecture, and early reliability issues. Here's the honest breakdown.
For exclusives & media: PS3 wins. For multiplatform performance & online ecosystem: Xbox 360 wins. For value: Depends on what you paid and when.
The PS3 is a legend for its library. The 360 is a legend for its infrastructure. Both defined a generation.