what's the best bike for on-road exercise? thinking 45 to 60 min rides at a time (for starters).

I recommend Giant Bike. It's a famous brand and has modern design. You can choose a bike depending on your need. Check the following link:

www.giant-bicycle.com

Finding the right bike for excercising is more about fit than anything else. Riding a bike that does not fit will cause pain and likely also knee and/or back problems. The for starters part indicates that you potentially want more, so I will recommend finding a good local shop that can measure you and find a bike of the correct size with the right angles for you. In my area I can recommend http://bicycleoutfitter.com/site/intro.cfm and http://www.cupertinobike.com/. Both are experts in finding and adjusting a bike just for you. Cupertinobike is perhaps a bit more for the hardcore riders. Please dont forget to buy a good pair of bike shorts. And for the saddle we are all different, but soft is not necessarily more comfy in the long run.