There are a lot of circumstances when catching a Pokemon. This includes your accuracy on throwing the Pokeball toward the Pokemon or the Pokemon's level itself. However, the developers of Pokemon Go also include the base capture rate for all available Pokemon in the game.
What are these base capture rate, you say? It is basically the probability of catching a Pokemon without having help like using in-game items. Also, these rates signify which Pokemon are the easiest and hardest to catch.
From Easiest to Hardest:
56% Catch Rate:
48% Catch Rate:
40% Catch Rate: