flight would be awesome, but i wouldnt choose it first. you could have so much fun with it, but it would be a selfish power. strength/duribilty id have to go with. its impossible to have one without the other, if you have muscles that can contract with enough force to lift tons and tons but they were as durable as a normal person theyd just tear themselves up in trying it. that or theyd snap your bones everytime you tried to use it. so that would be first.
you could still do alot of the stuff superman does, stop bad guys, save people from burning buildings and stuff. flight would be the most fun, but youd be pretty helpless if you tried to use it to directly help people.