You can take a look at the point below comparing Psychic Pokémon attacks between Slowking and Mew.

Mew: Psychic
Type: HP: 100, Attack: 100, Defense:100, Special Attack:100, Special Defense:100, Speed:100
Total Ability: 600

Slowking: Water & Psychic
Type: HP:95 Attack:75 Defense:80 Special Attack:100 Special Defense:110 Speed:30    
Total Ability:490

So you can compare the abilities of the two to see which Psychic Pokemon is better.

Read More: https://larvesta.com/psychic-pokemon/