15 miles (is this x2) is a fair way so you need an efficient, practical bike, the basic all-rounder.
Some of the sportier hybrids or flat-bar road bikes are good.
If road style drop-bars dont scare you off, there are some viable commuter road bikes.
A few pointers:
1. Avoid any suspension, you really dont need it and it adds weight and complication.
2. Have an idea of the largest tyre width you will use. On normal road, this is probably 32mm but in winter or on tracks and trails, it could be wider. Road-bike style commuters do it on 25 or 28mm with no problems. On MTB wheels, 1.5" is a good width for the road. Many bikes have limits to the widest tyre they can take.
3. Make sure the bike has threaded eyelets to accept luggage rack and fenders.
4. Pick a bike shop that can help you get the correct size in a suitable style.