A self-driving vanpool / paratransit service would likely be most optimal.
Single-occupant vehicles simply occupy too much lane space, especially at peak use hours. Transit routes are most efficient when structured as point-to-point. I've seen several commuter systems in which buses collected riders over a small area, then travelled to a (preferably concentrated) business / commercial area where riders disembarked. You limit loading/deloading and dwell times, and maximize time-in-motion (which increases net average speed). Most urban transit systems achieve net speeds little over a walking pace (10-15 MPH isn't uncommon) due to frequent stops and congestion.
The real killer with transit systems though is transfers. Having to wait 5-10 minutes (or 20-30) for a connection really extends your commute time. Realize that at highway speeds, that translates to 5-30 miles of commute distance, and even at urban speeds, 3-10 miles of lost travel distance. To say nothing of the stress of wondering where the bus/train is and when / whether it will arrive.
So: with an autonomous self-driving multiple-occupant vehicle, passengers could flag a ride, the dispatching system would match requests with livery by start and end points, and route accordingly.