How is the rating determined in the pokedex ?

rating = 0.5 * (round (rng * 20))

Explaination:
The above formula gives numbers bitween 0 and 10, in 0.5-Steps. The rng-element can easily explain how nidoqueen is better than umbreon and kingdra, as well as donphan being better than cloyster, or golem better than flareon.

The rating is given individually based on the Pokemon's role(s). If it's an attacker it's based on attacking, if it's a defender it's based on defending. If it's a great attacker but just not meant to be a defender it's not penalized.