Southwest Fisheries Science Center biologist Nick Wegner holds a captured opah, the first-ever warm-blooded fish. this is the biggest fish, much less a warm one i have ever freaking seen! [i don't include whales & such]
The Oarfish is a rare, solitary, and giant denizen of the ocean depths (arguably larger than a Whale Shark. (The as opposed to the Whale Shark, in the Guinness Book of World Records.)) and is a filter feeder, comfortable cruising at depths of 200 m.
Ocean Sunfish (Mola mola) are the largest bony fish in existence, measuring up to across and weighing lbs. Their favorite food is jellyfish. They have no tail. At night they often come to the ocean's surface for hours at a time, barely moving.