I predict a single speed mountain bike will make you very happy. I built one up last year with a 32x15 ratio (it fit perfectly without a tensioner) and I love it. It kills hills, and it actually forces me to slow down and take in the city a little more. I put it together just for the hell of it, thinking I wouldn't ride it a whole lot, but it has far exceeded my expectations. I've even taken it to work a few times (8.5 miles each way) and they were surprisingly excellent rides.

You can look at it right here : http://velospace.org/node/2696