They are all much the same service wise in the back, not very good! But they all appear to be equally safe and that's what matters.
If you think you can rack up 50,000 miles on a single alliance next year then join United Mileage Plus and fly United. That will give you the best upgrade chances on dirt cheap tickets, access to business class check in, lounges, and best frequent flier mileage accumulation on dirt cheap tickets. If, like >97% of the flying public you don't fly 50,000 miles a year, fly the cheapest.