I've had a few favorites...Here are some of them as well as why.
For slower songs, I absolutely love playing Moon on the Water. I came up with the my part on a bus ride on the way home from visiting my girlfriend back in 2008, just listening to the music over and over again since we had started work on the album. The E B E' B Bb lick that you hear in the song always gives me chills. Also, the chorus uses a favorite overused chord progression of mine that hides itself well enough that I didn't realize it for an entire year.
I also enjoy playing Sewer Surfin' a lot because for close to half a year, I didn't play anything on it because I couldn't comprehend what part I would possibly play. It's such a guitar heavy song that I had a hard time imagining what sort of place an acoustic piano would have in the music, and it just fell into place one day.