a bike with a rack where you can carry a bag or panniers to carry whatever you need for work, a helmet and a basic tool and an extra tube to solve eventual punctures, and a pump of course. (Wider section and good quality tires will make punctures less likely but it doesn't hurt to be prepared), i personally like a mountain bike for commuting, because you can also use it during the weekend for recreational riding or whenever you do some off road - hill riding, city or urban bikes offer you limited confort and less capabilities for anything other than commuting, p.s. my main bike is a road bike,