The giant Cretaceous marine lizard Tylosaurus. And yes, they're really just lizards, not some exotic extinct group like plesiosaurs or ichthyosaurs. The most recent studies place mosasaurs near snakes (which are also lizards, so please stop saying "lizards and snakes"), although previous studies have placed them closer to monitor lizards.
Anyhow, I couldn't decide between the closed mouth pose, which I find more aesthetically attractive and would have been a more common life view, and the mouth-agape-feeding-and-looking-badass version that better shows off the teeth. So I did both. For those of you who haven't kept up with recent revelations with mosasaur soft-tissue, it turns out that they had tail flukes and stiffer bodies than previously thought, swimming more like a shark than a giant sea snake as has been depicted in the past.
Edit: Updated the sternal elements, made some changes to the articular angle of the chevrons at the tail bend, and also updated the tail fluke to a slightly more contemporary interpretation.