New to this website and wow there are a lot of message boards here! I think this is the best forum for my question since it's pretty general. All things being equal, I was curious which domestic airline is the best for free first class upgrades?

That's the only thing I'm interested in and after spending several hours looking at all the programs I still have no clue which would be the best. Again, let's pretend that everything else is equal. I know in reality sometimes one airline is more convenient because of location etc but I'm just curious which is the easiest overall with upgrades. Thanks and I look forward to participating on FlyerTalk!