I was just thinking about this this morning. If I could have only one of Superman's powers, it would either be heat vision or flight (for obvious transportation reasons).
What about his humility and respect for life? Seems more developed than many people. In fact, I think this aspect was effectively used in one story. Something along the lines of someone observing Superman (or Superboy?) using each of his powers with the objective of being able to copy them for his own greed. When he was using his heat vision he used his superbreath to push away a bird that would have flown into its path. This trait was also "copied" with the result that the villian turned out OK.
OK, not much money in that!
Perhaps I'd go for super-intelligence? Then I'd learn how to spell "resign" and hand my notice in!
If enlarging the discussion to the rest of the DCU, I think I'd go for Flash.
As an aside, Superman can make use of his superspeed to travel in time. Has Flash ever been able to travel in time?