For a single 500'er you can't beat SFO-PDX based just on the length of the flight.
Otherwise, IAD-DEN is a great use at three, and JFK-LAX is a great use at five.
Keep in mind that if upgrades clear before you check-in you are only charged the direct distance between your origin and destination. So, while JFK-SFO is six, JFK-SFO-LAX is only five if it clears before check-in!