I think a yellow cab would be your best bet. You might have to wait in a line but likely not longer (or not much longer) than waiting for a Lyft or Uber to show up.
If you are arriving into T5, however, I would suggest Uber/Lyft as the taxi rank is at the other end of the skywalk and between escalators and elevators (and walking), any time savings is negligible.
-J.