A lightweight Tubus rack such as the Fly Evo or Vega Evo plus a single Ortlieb pannier. The rack (particularly the Fly) is so minimal you'll just leave it on all the time. And Ortlieb panniers attach and lift off in about two seconds. Removal is seriously just lifting a handle. This is my commuter setup.

Well, actually I have a Fly on my road bike and a Tubus Cargo Evo on my commuter. The Cargo is overkill for a commuter, though. I'll probably re-list it on ebay after I've found a Vega to take its place.