My commute's only 9 miles roundtrip so I could probably do it on a unicycle if I wanted. I've got three road bikes and one old Specialized Rock Hopper (the Bomber) that I bought for $40 at a yard sale. It's heavy, slow, and ugly. And I really hate flat bars, even with bar ends.

But I find myself commuting on the Bomber nearly everyday. I put a rack and grocery bag panniers on it and I just throw my backpack in it and go. No need to change shoes with the old-school nylon mtb clips and straps. It's just hop on and go. (Once I figure out how to put a set of drop bars and brifters on it, it'll be perfect.)

So, my two cents is: find an old all-rigid steel framed mountain bike. They stopped making them about 10 or 12 years ago, I think. Which is a shame, imo, because it's a do-it-all bike.