I went to Yellowstone five years ago, at the time I had a DA55-300 and a DA18-55 and a couple of manual focus lenses that I did not use much. The 55-300 did most of the work. I like to shoot wildlife, and Yellowstone is one of the best places on the planet for wildlife. A longer lens would have been better. I would have loved to had the DA*300 that I have now, if nothing else because of the extra speed useful early and late each day. And this is a place that you will want to be out from before sunrise to after sunset. There are also some fantastic geological wonders there. Don't limit yourself, have at least a good long lens and good wide lens. A few I would recommend, some have already been recommended:
DA* 60-250 plus optional HD 1.4x TC I don't have either but I've seen the results
DA* 300 plus optional HD 1.4x TC I can personally recommend the 300
DA12-24 I don't have but look very good
Tamron 10-24 Cheaper ultrazoom, very good as long as you're not looking into the sun
DA 15 Limited I just got this recently, excellent results, it would really be fun at Yellowstone, but a little less versatile than a zoom
Sigma 50-500, I have rented it once, it would be very useful at Yellowstone, a little on the slow side early or late. Renting was a very good experience to use a lens you need, and way to try a lens before buying too.
DA55-300WR Good long zoom, it has a slow focus that you can overcome by learning to quick shift first then auto focus. I got some very good shots with this when I was there with a K20D, with the K-30's low light ability you would do much better.
DA18-135WR good for everything but wildlife
Make sure you have at least one weatherized lens, or at least get a cheap rainsleeve.
Since the title of the thread is "If you had to choose one lens", it would depend a lot on what you're most interested in shooting, and what you may use the most after the trip. Of course your budget may or may not handle some of the expensive lenses I've thrown out there. I would say buy the best lens you can afford. If it is a wide angle, rent a longer lens, if you buy a long lens, rent a wide one.
Also don't forget extra batteries and the charger, and an extra SD card or two. Have fun, when are you going?