blue whale is the longest known, a certain type of jelly fish is by far the longest it is like 2.5x as long as a giant squid or more. i have this book called incredible comparisons that just compares things like that and has full illustrations for all of it. i havent looked at it in awhile but normally pick it up once a year or so. if anyone has seen those cross section books it is a lot like that. anyhow blue whale weighs the most and this type of jelly fish is the longest, but really id say its unknown as the ocean is pretty deep im sure there are some behemoths down there lurking somewhere