Which level 5 monster is the best?

In the early stages of the game, the most useful and effective units are the Rock Golems (although you might not have enough resources to upgrade them). Minotaurs join the battle a little later, or at the same time as the Rock Golems, but they are supported by the Eyeballs, so Minotaurs are very important as reinforcements.
In general, Gorgons are undoubtedly the strongest Tier 5 unit, but that doesn't necessarily mean they are the best.
There are no replacements for Birds and Minotaurs in terms of their roles, and in terms of power, Gorgons are ahead of the rest.