if you're looking for freeware, which, your selections seem to indicate, then I would recommend the TDS or Project opensky one. however, if you are willing to pay a little bit, you can't go wrong with the Qualitywings 757. and Avsim has a great livery for Allegiant using the Qualitywings model. likewise, if you want to spend a little more, there is the Captain Sim version of the 757, with a more complicated cockpit, but also has an allegiant livery native to its livery manager.