The Megalodon is my personal favorite, the idea of a shark the size of a schoolbus is pretty cool.

As for those with a chance of actually being discovered... that's a tricky question. As your own link to wikipedia shows, several animals formerly thought of as cryptids are now widely known to exist. Most famously the Giant and Colossal Squid was considered to be an impossible creature even as recently as 2001, it wasnt until 2004 that one was successfully photographed in its natural habitat (kubodera).

I think the cryptids with the best chance of being found to exist are those which don't have any real supernatural connotations or violate too many physical laws. For example the common school demonstration of how an ant over a certain size would be crushed by it's own weight leaves most giant insect cryptids firmly in the realm of fantasy while anacondas of up to 10.5 meters have been found in the wild, leaving it much more likely that a particular anaconda may at some point grow to be significantly larger (if unhealthy and unlikely to survive) (Murphy JC).

The phoenix is a natural predator of the Elemental Plane. The wondrous bird is about 18 feet long with a sleek, feathered body that dances with bright flames. Its colorful feathers range from yellow to orange to red. A crest of spiky feathers runs down the center of its back, and its wingspan stretches out 30 feet. From birth to death, it spends its life hunting through the air -- in fact, it lacks any legs that might enable it to land. Even after death it often rises again from its own ashes to continue the hunt.

The phoenix is a fearless, aggressive predator of aerial prey. Its only direct form is a vicious bite, but the secondary damage from its flaming aura and ability to burn help to bring down its prey quickly. A phoenix prefers to attack from above and behind with surprise, but follows up its initial attack by remaining close to burn its victims while continuing to strike with its beak