DWF already gave a good reply for this. Caiman is indeed inferior. The flanks are quite soft and are great for belts, but the centre scales are full of osteoderms that make them very stiff. They also don't take dye very well.
Between alligator and crocodile, it mostly comes down to preference. Alligators have a larger head and tail, giving better cutting yields. I find the head to be really nice for wallets.
The porosus crocodile (saltwater crocodile) is generally considered to be the 'premium' leather compared to the others. Other than that, crocodile and alligator are generally considered interchangeable.