Humans actually. "The brain to body mass ratio in some members of the odontocete superfamily Delphinoidea is second only to modern humans, and greater than all other mammals"

https://en.wikipedia.org/wiki/Cetacean_intelligence

But their neuronal density is much lower.

IIRC their high glia/neuron ratio is though to be an adaptation to sea life.

Neuron interconnectivity might be more important than plain neuron count. So that could be an advantage.