Up until this past February, the largest animals I'd seen on a dive were the whale sharks off Isla Mujeres in Mexico. It was so exciting to get to swim with them!
Then in February, I was diving off French Cay in Turks & Caicos when our group saw a humpback whale and her calf. We all swam out into the deep blue beside them for several minutes, and it was an incredible experience. Magnificent animals! (Photo of one of the two here.)