I climbed Denali this year with RMI. Our group was on a parallel trip to ones run by Mountain Trip, Alpine Ascents as well as Alaska Mountain School (AMS). All of these trips were well organized and safe, and I am confident that you would be well served by any of these outfits.
I have experience as a customer with both RMI and Alpine Ascents, and would very highly recommend either one. That said, what I saw of the AMS team, they appeared to be very good as well.
Some useful links:
Another point to keep in mind is that much of the success or enjoyability of a trip is down to the individual guides involved. Since you're spending the same amount of money for all outfits (around $5000), you might want to pick the trip based on the guide leading it. There are some very well reknowned guides that work for these companies, including Dave Hahn (RMI) and Vern Tejas (Alpine Ascents) - do a google search on either one if you haven't heard of them.
Whichever outfit you choose, good luck with your climb and train hard! It will be the trip of a lifetime!