Right now my main "grab and go" is a single speed hipster bike built up from a 1980s road frame. It's totally inappropriate, but I love riding it. If there's weather or cargo involved, including trips to the store, then it's another old bike, but with a big basket, fenders, and 3 speeds. As winter approaches, I'm looking for an old beater to take my studded tires for winter use.
If you're wondering what to get, my advice is first and foremost to get something that you will enjoy riding. A bike that can be equipped with typical commuter attachments (rack, fenders) is a bonus. Gearing depends on the actual terrain of your commute. It has to look good.