The higher on the list the better the enemy. The more in the left the better. Height has priority over length, tho.
I am ranking Yog-Dzewa as if it didn't spawn any minions at all. Ranked those separately.
A way to deal with them is to have them follow you to a door. Once you go through the door and they enter it, you're guaranteed to hit. They usually die on the first hit. Just make sure you know what's behind the door :-)