1.) The Whale Shark (Rhincodon typus)
A well known candidate, the Whale shark, is the largest known fish in the ocean. It is not a type of whale, but instead is called this because of its massive size. The average size for an adult Whale shark is about 12 meters long, and can weigh up to 40 tons. The largest recorded Whale shark is 20 meters long, bigger than a school bus. Despite its size however, it generally feeds by filtering small organisms (plankton, schools of fish) through its mouth.
2.) Basking Shark (Cetorhinus maximus)
Another large filter feeder, and the second largest fish in the ocean is the Basking shark. It can reach up to 8 meters in length and weigh nearly 19 tons. The Basking shark is well equipped for filter feeding as it has an enormous mouth and its gills have rakers that pick up small organisms.
3.) Greenland Shark (Somniosus microcephalus)
The Greenland shark is a unique in many ways beyond its sheer size. They weigh about a ton on average, and can grow to a size anywhere between 5 and 7 meters. They feed on anything they can sink their teeth into, but often get food scavenging for corpses. On average, they swim at a speed of 0.3 meters per second (0.76mph), and max out at about 0.9 meters per second (2 mph). The Greenland shark is also the longest known living vertebrae on earth, aging possibly over 400 years old.
4.) Great White Shark (Carcharadon carcharias)
Possibly the most well known fish due to pop culture, the Great White shark comes in at 4-10 meters long, and weigh around 1-2 tons. They are very aggressive and great hunters, preying on any fish or animal that come into its path. It's been known to launch itself out of the water to catch birds as well. They are essential to ocean life however, as they control many species' populations.
5.) Beluga Sturgeon (Huso huso)
The Beluga Sturgeon is an interesting species as it is found both in freshwater and saltwater, and can live up to and sometimes over 100 years old. Being part of freshwater ecosystems, they are also known to be the largest freshwater fish known today. Beluga Sturgeon migrate from saltwater to freshwater for reproduction. They grow to about 6 meters long, but are sometimes found to be as long as 7 or 8 meters. Normally they weigh in at around 1-2 tons as well. Generally, they feed on fish smaller than them, although sometimes they prey on bigger animals, such as baby seals.
Ocean Sunfish (Mola mola) - Largest Bony Fish
Ocean sunfish are much different from freshwater sunfish. They can grow beyond 3 meters long (and tall) and weigh anywhere from 1 to almost 3 tons. They are considered the largest "bony fish" recorded. Their diet however, mostly consists of jellyfish.
Tiger Shark (Galeocerdo cuvier) - Most Agressive Fish
Another predatory shark on this list, the Tiger shark has an extreme diet. It will eat literally anything, sometimes even inanimate objects, such as tires and metal. Called so due to the strips on its back, the Tiger shark reaches lengths up to 5 meters, and weighs around 1 ton. Tiger sharks are known to be vicious predators, coming second only to the Great White sharks. Although they can't eat some of the large animals as Great Whites, they can be much more aggressive.
Black Marlin (Istiompax indica) - Fastest Swimmer
The Black Marlin is known as the fastest, and another one of the larger, fish in the ocean. It can reach speeds northward of 35 meters per second (80 mph). Sizes vary, ranging from 3-4 meters (sometimes longer), and averaging a weight just shy of a ton. Their size, shape, and speed make them popular in sport fishing as well. They feed on other small fish, including mackerel and tuna, using their long bills to attack and weaken them. Their speeds are, in part, attributed to their size, shape, and ability to fold their sail fin.
Giant Oarfish (Regalecus gleans) - Longest Bony Fish
The Giant Oarfish is not so heavy as it is long. Weighing in at only a quarter of a ton, they can reach lengths of a record 11 meters long (almost the length of a full sized school bus). The only feed on small organisms, such as plankton and shrimp.