I normally commute on my track bike..which out of all my bikes is actually the least like a typical "commuter" but its fast and fun to ride. I travel light and can easily fit all my stuff in a backpack, so I don't need a dedicated commuter all decked out with racks pannier's and such.
I have a fendered fixed gear beater for rainy days, and a SS mountainbike for snow. Once in a blue moon I'll ride my geared road bike in.
If I could only use one bike, I suppose I'd go for something that was a little more utilitarian, maybe something like a touring rig, but personally I think that would get boring day after day.