Unfortunately, at some point in time you will run out of companies and options. As Ken had noted, all the majors are really the same. There are only three big conglomerates now. If you don't want to rent from Budget, then you probably should skip Avis as they are owned by the same parent corp. The same employees help renters of both companies. They just need to swtich from one system to another. The same is really true of Dollar, skip Thrifty and Hertz. If you have a bad experience with Alamo, forget about National and Enterprise.
I can understand the principle, perhaps from an emotional standpoint, but for us it really comes down to price because at some point or another they are all going to screw us over in some way. They are all equally incompetent with employees that don't really give a crap and are just taking a paycheck.