For me its:

Blue- "Undone (The Sweater Song)", Matt's falsetto during the verse, Brian's backing vocals during the chorus and Brian and Matt's Falsetto during the outro.

Pinkerton-"El Scorcho", Brian's falsetto in the intro, Brain and Matt's backing vocals in the chorus and Matt's falsetto during the bridge.

Green-"Photograph", Mikey and Brian's falsetto during the intro and outro and Mikey and Brian's backing vocals during the verse.

Maladroit-"Keep Fishin'", Brian and Scott's backing vocals during the verse and chorus.

Make Believe-"Hold Me", Brian's backing vocals during the verse and chorus.

Red-"Pork and Beans", Brian and Scott's backing vocals during the chorus.

Raditude-"I'm Your Daddy", Brian's backing vocals during the chorus.

Hurley-"Memories", Brain, Scott and the Jackass' cast backing vocals during the chorus.