Any reliable bicycle
If you have to buy a bike, there are three or four kinds of bikes sold to adults in most North American shops: Mountain bikes
Mountain bikes
If you are only using the bike for commuting, forget about a mountain bike. Mountain bikes
Of course, some don't really care about this -- they get mountain bikes to project the image that they dabble in off-road activities, or are a little on the wild side -- kinda like your average city/suburban SUV driver.
Still, they are tough, and in certain commuting situations -- through parks, potholes, or curb-jumping -- they're your best bet.
Touring bikes
These are the speediest option, thanks to their light frames and thin tires. Up until the mid-80s they were commonly known as "ten-speeds," but most newer models have 24 or more speeds.
If your commute takes you on long stretches of road without any interruptions, and you can build up a lot of speed, you may want to get a touring bike
However, if you are driving through city streets and have to deal with red lights and frequent intersections, a touring bike is probably not the right choice.
Touring bikes
I have also found them to be slip-prone when encountering sand on pavement, which is a common situation in the spring in northern climes.
Hybrids
Hybrids
Granny bikes
I don't know if this type of bike has an official name, but this is what I call them.
This type of bike is the descendent of the solid model you might have seen an elderly grammar school teacher riding in the 1950s. They have curved handlebars, wide seats, a simple three- to ten-gear transmission, and mud guards. A few even come equipped with lights and baskets. Recent models tend to be black and ruggedized a bit, with wider tires and stronger frames, kind of like a hybrid lite.
They are subtly ubiquitous in university areas, and even overseas. They are also entirely practical and maybe even right for you if your commute is a short one, or you don't need to go that fast.