Success for an animal basically means the most babies.
Sharks have been around much longer with minimal change, this shows that they have been a very successful group of animals for a couple hundred million years. Looking at past success/success overtime I would say the shark takes it.
However, currently, mammals are wider spread and adapted to more varied environments (marine, fresh water and terrestrial). As a result they are more likely to continue to be successful as the environment changes. So for currently successful and likely to continue to be successful mammals will have to take it.
