If you're only going to ride 9 miles each way, your options are unlimited.
You could get a halfway decent road bike or a hybrid.
They come with multiple gears which means, you'll have to shift. This shouldn't cause you any concern. a properly adjusted bike will easily shift gears with a simple click of the shifter. Dropping a chain should be very rare occurrence, in fact it should never happen during a commute.
You gould get a singlespeed bike.
Lot's of people ride them. There is a certain simplicity about these bikes that people enjoy, and it kind of puts the fun back into riding a bike. The downside is that you only have one gear to do it all. Whether it's climbling hills or sprinting, your performance will be limited by the bike. But this isn't really a big deal. If people can race singlespeed mountain bike across the continental divide ~ 2,700 miles from Canada to Mexico in a 2 week perioud ~ you can ride a singlespeed on paved roads for 9 miles at a time. If you opt for this kind of bike, don't allow yourself to get sucked into the trend of riding a fixed gear or "fixie". You'll want to be able to coast without pedaling, and fixies won't allow you to do that.
You could get a mountain bike.
The bike geometry and the upright riding position appeals to a lot of people ~ myself included. In fact I commuted to work on my mountain bike for 7 years ~ riding 15 miles each way. I put skinny tires on it, and I was just as fast, if not faster than most people on road bikes. I used to average 22 mph on the road.
You could get a BMX bike.
Granted, this would be the most difficult and inefficient option, but it certainly can be done. You'd be doing a lot of standing pedaling, which flies in the face of the road bike / sit and spin crowd. But I look at it this way ~ there's a whole world of bike handling that begins to open up when you can ride out of the saddle for extended periods of time. I can't remember her name right now, but I read about a girl in a mountain bike race who rode 50 miles off road, without a seat on her bike. I can't remember the exact reason why she did it. I believe it had something to do with it being misplaced or lost. The race started, and so she decided to just ride without it.
As you can see, you can ride anything that you want. Don' t allow other riders from niche groups try to place artificial limits on you. If your goal is to get to your destination in the shortest amount of time, then there really is one bike for that ~ a multiple geared road bike. But if your goal is to just cover the distance, then you can ride anything else. Heck, you could even do it on a unicycle if you wanted to. I'm sure there are people out there who do it.