The most useful monster.

Considering your response, the updated ranking looks like this:

1. Genie (18)
2. Vampire (13)
3. Black Dragon (7), Magic Dragon (7)
4. Angel (5)
5. Mantis (4)
6. Water Elemental (3), Ghost (3), Fairy (3)
7. Titan (2), Minotaur (2)
8. Tiger (1), Leprechaun (1), Devil (1), Thief (1), Cyclops (1)

Changes: Magic Dragon is now tied with Black Dragon. Angel has significantly risen from 7th to 4th place.