I meant to coment for a while, but all these lists have a huge flaw, they don’t take in consideration the amount of waves and enemies in the levels. Nor the benefits regarding loot, exp and such.

For example, if you are farming Blues and want to do it as fast as possible in one go, the level you should be playing is the 8-7.

8-7 is a lvl with 4 waves. 3 full blue, 1 full green. Each one of the blue waves can spawn between 3 or 5 monsters. So that’s 9-15 blue enemies to kill.

8-5 is a lvl with only 3 waves. First 2 can spawn 6-10 enemies and the last one always 3. That makes a 9-13 chance of blues.

Its not only about the possibility of more enemies, 8-7 also rewards higher exp, food and iron.

It may sound like little, but after repeating the level a dozen times, that gain can be a huge difference in both time spent in that and final gains. In other words it a more efficient choice in all fronts.