What kind of bike is "best" depends on your commute, and your needs.
Steel MTB: if you get an older good quality one, they're awesome. Strong, comfortable, easy to work on, good for shorter commutes or nasty weather. Also good for hauling trailers or heavy loads, not so great for panniers if the chainstays are short.
Hybrids: great all-arounders. They come in lots of variants, but basically, the better ones are road bikes with flat bars. Great for city riding, gives you a comfy upright position to see and be seen in traffic. Not the most efficient, but faster than a MTB.
Steel Touring or Cyclocross bikes: this is (for me) the sweet spot. Strong, fast, comfortable, made for long rides hauling panniers. This is what I use. Kind of expensive if you're buying new. Not the fastest, but does awesome at almost everything else.
True road bikes: I'd only recommend these if you're commuting long distances and you aren't carrying lots of cargo. They're fast and light, but the faster and lighter it is, the less suited it is for commuting. Most high-end racing bikes won't fit fenders or tires wider than 28mm. If your commute is smooth and long, and you can leave changes of clothes at work, this works well.
Obviously there are exceptions to these, but this is basically what I'd recommend.