Believe it or not, the ES is most probably the best in terms of cheapest future. On a given day that the ES will move 10 points during RTH, the NQ can easily do 50-60 points. Consider that 10 points for ES is $500, but 50 points for NQ is $1000. YM is the same story. It doesn't matter that its only $5 per tick because it will blast through dozens of ticks in no time.
Looking at this another way, consider a stop of $100. For NQ, that is 5 points, and this means your stop can hit in about 1-2 seconds. For the ES to move $100, which is 2 points, it generally takes more time to do this. The swings are smaller. When the ES does move 2 points in only seconds, its generally quite significant, or at least a move orchestrated to run many stops, which isn't easy to do.
Looking at this another way, consider a stop of $100. For NQ, that is 5 points, and this means your stop can hit in about 1-2 seconds. For the ES to move $100, which is 2 points, it generally takes more time to do this. The swings are smaller. When the ES does move 2 points in only seconds, its generally quite significant, or at least a move orchestrated to run many stops, which isn't easy to do.