What's your budget?
Very limited? Vintage road frame with rack/fender mounts. <300
Somewhat limited? Used bike off craigslist. <400
Not so limited? Used bike from the LBS, or a ~500 dollar new hybridish bike.
Skies the limit? Cyclocross/touring bike, start around $800, can cost $4000+.
Sort out what features you want, drop bars are far more comfortable then flat, and depending on riding position can help decrease wind resistance (most important for speed.)
Most commuters want/need rack/fender mounts, ability to fit 32-38C tires AND fenders, durability over weight, and gearing appropriate for the terrain. Ride a few bikes that match your short list, spend 15-30 mins on each bike to see how your neck, hands, and back feel.
Saddles always hurt on new bikes, it can get expensive finding the right one, budget for that. Avoid lots of padding, sounds like it's good, but it causes a lot of pain unless you're tooling around for 2-3 miles tops.
Oh, unless your bike is locked inside a secure building, keep it cheap, nice bikes tend not to last very long locked up outside.