I'm sorry, I was unable to decide between Halo 2, Half-Life 2, Grand Theft Auto: San Andreas, and Ninja Gaiden. They're all so awesome, I had to vote for all of them.
I didn't vote for Ratchet and Clank though, because that's not the best game of 04. It's completely the same exact thing as the first 2 Ratchet and Clanks, so I personally count it as the best platformer of whatever year the first R&C came out in.