The whale shark, the largest living fish, if you use the term "fish" to describe the polyphyletic group of aquatic and marine vertebrates. To render the word monophyletic, you need to include the Sarcopterygians, i.e. lungfish, coelocanths, and tetrapods. So, under the more accurate terminology, mammals are fish, too, placing Rhincodon typus somewhat down the list, perhaps around 10th place, after the rorqual whales in the global body-size contest.
Truly large animals always seem to be herbivorous (Diplodocus, Apatosaurus) or planktivorous (Balaenoptera, Rhincodon). So maybe next week I'll have something that (contrary to almost everything else) is not listed as "Harmless" by fishbase.
The above picture is nice, but doesn't really show the whole animal's size very well.
Also, what the hell am I talking about with my paraphyly thing? Look at this:
Image credits here, here, and here