I think a light touring/cyclocross is the best bike for commuting (most light touring and cyclocross bikes have similar frames). With commuters, you need a good balance of performance, function and comfort, and cyclocross bikes fit the bill perfectly. They're more stable than straight road bikes, but nearly as fast. Cyclocross bikes are much faster than mountain bikes. They don't have the cargo capacity of a true loaded touring bike (like Trek 520), but with panniers they have plenty of space for commuting gear (clothes, etc.)

Though some people swear by them, I don't think a loaded touring bike is a good choice for commuting unless you only want one bike and actually plan on doing loaded tours. Loaded touring bikes are slow, and less stable when they aren't fully loaded because of the geometry. Before I got my bike, I tried out the Trek 520 (unloaded) and it felt less stable on fast decents than my cyclocross.