Which rental company? DEN isn't any different from all the other major airports in the U.S.: if you have status with the rental companies that allow self-selection (Hertz, National/Enterprise, Avis, etc.) and book the appropriate class (usually intermediate or above), you can pick your own car from the lot. If you don't, you can ask for a specific vehicle when you go up to the counter, but be prepared for pushback and upsales, especially if what you want is in a different class to what you've booked.