Which level 5 monster is the best?

Minotaurs and Gorgons are the best in terms of combat)) but not always in terms of tactics)) because Rocs can be more useful due to their flight, or Liches due to their cloud)

In general, almost all level 5 creatures are good))

Minotaurs have good stats (almost all of them), and their appearance!... it's just great) much better than in other parts))
Gorgons have good stats, and their Death Gaze is very good)
Dendroids are good due to their defense, decent survivability, and ability) and their appearance is good) better than in the 5th game...
Fanatics are very appealing) they are good archers and have an excellent appearance)

Magma Elementals and Liches have a good appearance)
And the Broodlings look like the Shadow Lords from the 5th game)) also with a whip)) or rather, the other way around))