My Flight Memory is only accurate back to 2004 or so, but as of right now, CO is in second place with 20.6%. UA is first with 27.2% (In reality, I'm probably missing 200+ UA flights from 2003 and back, as I only entered those that I could remember...), and US is in third with 20.2%.

For 2009, CO is 60% and NW/DL is 40%.