Considering all the indicators, not only the monster's stats, but the entire castle:
The strongest are Titans. Why not Archangels? They cost the same in gold, but: in the tower, it produces stones, while in the castle, it produces ore and wood. This means that less resources are spent on building, upgrading, and hiring in the tower.

In general, the overall rating is as follows:
Archangels
Titans
Black Dragons
Archdevils
Golden Dragons
Behemoths
Phoenixes
Hydras
Spectres