Don Gilmore's post is great about saying Chester was working his ass of on Hybrid Theory and was the main driving force behind that album. Mike didn't start his semi-producer role until Meteora.
Chester wrote a lot of stuff over the course of the band. The core songwriting units are Chester, Mike, and Brad. Rob, Dave, and Joe are like the icing on the cake, especially later albums. Sure, Mike comes up with song demos, melodies, and shit, but Chester is the main lyricist and a huge writer.