The answer to this question depends on how you define "success rate." If you're talking about survival rates, then bacteria have an incredibly high success rate. They are incredibly adaptable and can survive in a wide variety of environments, even extreme ones.
However, if you're talking about reproductive success, then the answer might be different. For example, insects have a very high reproductive rate, with some species laying thousands of eggs.
Ultimately, the "highest success rate" is a subjective measure that depends on how you define success.
