I vote for Bigfoot, because the sightings have been reported in many countries, usually in remote, forested areas in places with cold climates. There were, and probably still are, undiscovered homo sapiens tribes in the South American rainforest, so why shouldn't there be remnants of another human-type species somewhere?
As for Nessie, there are many things in the deep ocean we haven't yet discovered, but the loch is a deep lake and there is no current access to the sea. There would have to be a breeding colony in the loch, quite a few Nessies, and you would think the latest technology would have spotted one or two. However, it would be fun if it turned out to be true.