Either:
a roadified rigid MTB, or
a tourer
If it's to be used all year round, it'll need mudguards. Lights are useful (I keep mine on all year, since I work nights. And even when I work days I never know what time I'll be finishing.), and a dynamo system is perfect for commuting, since you don't want to be messing about with batteries every week. Depending how much stuff you need to take to work, I'd say a rack and a pannier (or two, in my case) are also essential. Finally, if your commute is reasonably flat, you could take the front derailleur and a chainring or two off to make the whole thing simpler. Or possibly even use a singlespeed or fixed wheel bike to reduce maintenance time.