Bing is the easiest and cleanest site to search for flights. I travel a lot for work and am always looking at flights, Bing is the way to go.
That being said, they do not have prices for Allegiant, Virgin, or Southwest. Nobody has the fares for those three except their own sites.
I am betting you will find your cheapest flight on Allegiant but do realize you can’t choose your seat unless you pay, pay for carry-on, no free water or soda, rarely are they on time. I find it is much smarter to pay the extra money up front.