I have to go with Leviathan

Leviathan didn't even exist, plus it's just an overgrown bug

Plesiosaur is pathetic compared to the rest.

Megalodon is just a giant mouth; it's easy to beat it if you have a long body that can wrap around it and squeeze the life out of it, because it can't do much to get itself free. Unfortunately for Mr Shark, that's exactly what the Big Octo and Leviathan have.

So finally, it's Big Octo vs Leviathan...and Leviathan is clearly the winner. No questions. It's a fucking monster with all sorts of crazy fantasy features that allow it to basically kill anything.

The octopus is too fragile, plesiosaurs are said to be the prey of megalodons, and leviathan is rather undefined.
Megalodon, pending specification of 'leviathan'.

release the kraken!

i'll pick the one not extinct or fictional. Giant squid sightings off the coast of Mexico baby! Check out the episode of monster hunters, i think they actually get a crappy/half focused shot of one.
sigh, never any good shots of monsters like that.

LEVIATHAN!!,

i know i see it in the game,,

hmmm. i think that game is Final Fantasy,
especially in Dissidia Duodecim,
Leviathan is one of the summon stones!!

and look pretty cool and scary *o*

Megalodon was a scary ass shark. .____.

couldn't see the Leviathan link but if it's anything like my experience from Gears of war then I'd go with that 1 xDD

well i don't know about deadliest, but the leviathan scared me the most O.o