Have you realized yet that some Pokemon are stronger than others? If you haven't, yes it's true. As much as you want your maxed out Vaporean to hold that gym, it's not going to be enough if one of these 10 Pokemon make their way into battle against you according to The Silph Road. From unbeatable to super strong here are the top 10 Pokemon overall in Pokemon Go...
Just missing the list by one position is the much coveted Gyarados and other Pokemon featured Poliwrath, Flareon, and Slowbro. I'm not too surprised at the list as every unbeatable gym I encounter seems to have a Snorlax guarding it. I haven't had a chance to encounter any legendaries protecting gyms yet though...how about you?