After commuting on an MTB, then a road bike, then an MTB set up for pavement, I finally settled on a cyclocross bike as the base for building what I think to be the perfect commuter... well, if it had a front disc brake it'd be perfect, but close enough is good enough!
I like drop bars in and around traffic.. I find the lesser width, and having your hands at the extremities of the bike make it easier to judge which nooks and crannies you can and can't fit into.
Although the spec is rather modest (compared to my 'real' road bike and MTB) I love riding this thing. Which I think is important, if you don't like riding your commuter bike, you won't commute as often with it.
Cannondale Cyclocross frame with Ultra X front fork (25mm suspension with lockout)
Tiagra groupset (9 speed, 12-26T cassette) and cheapy compact crank (34/50)
Maxxis Overdrive 700x38c tyres, with a set of Mr Tuffys installed
SKS fenders, and Topeak rack and quick-release bag with expanding panniers
Light & Motion Commuter 10W halogen light