I I chose Freddie, but that's because my favorite song is Play The Game. But Brian is the best hard rock/metal song writer (Hammer To Fall, Tear It Up, Put Out The Fire, Headlong, I Want It All), and writes great ballads, but then again, Freddie can come up with things like Was It All Worth It, Princes Of The Universe, Ogre Battle, and The Hitman (which Hitman might be more of a metal song than any of Brian's!) plus all the ballads he's written. John wrote some funky stuff (Back Chat) and some great ballads (You're My Best Friend) and some nice mainstream rock tracks (I Want To Break Free), and Roger was always a great mainstream rock (Calling All Girls), pop/rock (Radio GaGa), and rock (I'm In Love With My Car) writer.
Freddie was by far the best songwriter in the band. His input into the other's songs would have been massive, we know of stories with Freddie taking the other members songs and changing them.
It's tough but I had to go with Freddie of course. I still say that Brian's top five or so songs are as good as anything Freddie ever did, and I still feel that John Deacon had the best lyrics, and I think Taylor's overall catalogue is equal by his quantity of great songs and his various genious moments that occur in no less than a 3rd of his songs.
Freddie had the boldest punch in his riffs, piano, vocals and wild musical tantrems that went on particularly in the Queen I and II, SHA and ANATO periods. His lyrics seem to have the more poetic ambiguousness and big words with an undisclosed source inspiring the song. His grand classic piano creations and ability to get huge riffs out of Brian that I later realized how much must have come from Freddie when I heard the consistancies in solos and what not on the Mr. Bad Guy album. I just think Mercury had the more overall creative genius in him.