In terms of spawn probabilities, sheep are the most common passive mob, while creepers, skeletons, zombies, and spiders all have the same probability but the larger sight/pathfinding range of zombies makes them much more commonly encountered.
This is also based on the game's code itself and analysis made with MCEdit and my own extensive experience (my own mod keeps track of mobs killed in the current play session, with zombies virtually always at the top by a large margin) so this can be seen as an authoritative answer:
The first number in each entry is the weight, 12 for sheep, 8 for cows and the second and third are the min-max group size; zombies have two entries since zombie villagers are considered to be a separate mob but they are really the same (as they are in older versions; 1.6.4 only has a single entry with a weight of 100, otherwise, aside from the addition of witches in 1.7 there have been no changes up to at least 1.12, the most recent version I could find source for online, but I don't see any reason why this would have changed. Various biomes also have additional mobs; for example, taigas have a wolves with a weight of 8 and rabbits with a weight of 4, so even in a taiga-only world they still rarer than most other mobs). Also, while slimes have the same weight as other common mobs there is an additional failure rate applied to each individual spawn attempt (50% near the surface in swamps and 90% in slime chunks, so even if they could spawn in any chunk they would only spawn 10%-50% as often).
In terms of spawn probabilities, sheep are the most common passive mob, while creepers, skeletons, zombies, and spiders all have the same probability but the larger sight/pathfinding range of zombies makes them much more commonly encountered.
This is also based on the game's code itself and analysis made with MCEdit and my own extensive experience (my own mod keeps track of mobs killed in the current play session, with zombies virtually always at the top by a large margin) so this can be seen as an authoritative answer:
The first number in each entry is the weight, 12 for sheep, 8 for cows and the second and third are the min-max group size; zombies have two entries since zombie villagers are considered to be a separate mob but they are really the same (as they are in older versions; 1.6.4 only has a single entry with a weight of 100, otherwise, aside from the addition of witches in 1.7 there have been no changes up to at least 1.12, the most recent version I could find source for online, but I don't see any reason why this would have changed. Various biomes also have additional mobs; for example, taigas have a wolves with a weight of 8 and rabbits with a weight of 4, so even in a taiga-only world they still rarer than most other mobs). Also, while slimes have the same weight as other common mobs there is an additional failure rate applied to each individual spawn attempt (50% near the surface in swamps and 90% in slime chunks, so even if they could spawn in any chunk they would only spawn 10%-50% as often).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?