I ride a rigid mountain bike with an Xtracycle attached. I swapped out my knobbies for a pair of the Specialized Nimbus tires. They get me through the off road parts, and let me still sail on the roads.

It's only about $40 total to get the front and rear tire changed to the Nimbus, or any other similar tire. I highly recommend it.

The best bike for your commute is the one that gets you there safely, reliably, and happily.