What's the best bike for commuting? This is a heavily debated topic and many people do have preferences.
There are four major classes of bikes that you might want to consider.
1) Classic Road Bike
2) Relaxed Road Bike
3) Cyclocross Bike or Mountain Bike
4) Foldable Bike
The classic road bike is often the quickest. It will allow you to get to and from work the fastest way, and it would be easy to extend your ride home from work to make it into a real ride. However, for some people just want to get to work without being sweaty, they don't mind if it isn't as fast. Also, road bikes often don't have room for racks or panniers to hold your things.
The relaxed frame road bike is a big hit with many people. The geometry is more relaxed than a standard road bike, so your ride will be more comfortable. These relaxed frames often come with ways to attach racks and panniers so carrying cargo is much easier. However, these bikes are not quite as fast as the traditional road bike and they are quite a bit heavier.
Cyclocross bikes and mountain bikes are almost the same. They both allow you to ride off road and take a more versatile path to work. These bikes generally are good all around bike that allow you to ride on any surface. The mountain bikes will be quite a bit slower than a cyclocross bike though. Cyclocross bikes have space for racks and panniers more often than mountain bikes. Also, you may not want to go off road while it's wet out.
Foldable bikes are an interesting choice. They are by far the slowest. However, like I said before many people don't care how quickly they get to their destination. If you want to take the subway and just ride your bike a short distance than a foldable bike is the best option.
I would say that the best bike for commuting is cyclocross bikes or a foldable bike. These seem to be the best options, for me at least. What is your preference?