I've seen everything from a unicycle to a $4000 carbon fiber road bike in the rack at my office, but most people seem to ride mountain and hybrid bikes. Touring bikes and cyclo-sportive type bikes (light road bikes with fenders) would work well for that distance, but so would a lighter hybrid. Take traffic and road conditions into consideration. If you're mainly riding in heavier traffic you might want something a little more upright, and if your local roads have the same monster potholes that we have in Saskatoon, beefy tires and wheels are worth looking at.