A giant squid has never been caught on film alive. Measurements from the sucker-marks left on Sperm Whales has indicated that the beasts may grow to lengths approaching 100ft long, and while most of that is from the two long tentacles, that is still quite impressive.
Megalodon's aren't very likely to exist anymore, though there may have been a few left as recently as 10,000 years ago. Human's would be their only enemy, with a pod of orca's also being a possibility, but man, they would do serious damage to the populations of smaller whales, seals, etc...imagine a schoolbus with a mouth at the front of it cruising around and you get the idea of a Megalodon...
I recall reading something about partial carcass from a giant octopus, much, much larger than the one discussed in this article. This thing was giant-squid sized, if not bigger, but the remain was so incomplete that it was never officially classified..let me see if I can find it...
I have to admit, I was a "mystery animal" junkie growing up, along with dinosaurs, sharks, etc.....