Deepest spot in the ocean, I'm more likely to see things hardly anyone else has seen before. Added bonus of possibly spotting rare sharks like the Goblin, Frilled, or Ghost sharks. Basking Sharks are a bit...scary because of the way they feed though, I once saw a picture of a diver that almost got sucked into ones mouth. 0_o They're gentle though so they probably wouldn't snack down on a person if such a thing happened.
I love sharks, they're so widely varied in appearance and they haven't changed much over the course of time so it's almost like looking at history living and breathing right in front of your eyes.