What criteria to look for when placing pokemon to defend gyms?
There are 3 main criteria: The first being high hp, for obvious reasons. The second criteria is charges that charge fast and have a high base damage to take full advantage of the 1.5 attackrate. The third one is a high Defense-DPS, explained in depth here. But to simplify it: a high base damge is generally better.
The highest Tier actually isn't simply the highest hp Pokémon - it is more of a coincidence that the high hp Pokémon also have a higher base damage.