Blissey Is Insanely OverPowered
Outside of legendary Pokemon, slaking and snorlax, blissey is also one of the best Pokemon found in all of Pokemon GO. On top of being one of the rarer Pokemon to find or obtain, Blissey is another Pokemon with a rare combination of Stamina and Defense that sees it have a Maximum combat power Level of 3219. And this is why you see so many people, who are lucky enough to catch or hatch a few of them, leave it at Pokemon Gyms.
Because of its immensely bulky Stamina and Defense Stats, blissey makes for an excellence gym defender. It actually is one of the toughest gym defenders in all of Pokemon GO, right up there with both slaking and snorlax. (It is not known as the Queen of Gym Defenders for no reason!) And if it is at a high level, Blissey is almost impossible to defeat with just one gym attacker (perhaps even more). Thus, Blissey is widely considered to be, at least in this Pokemon Game, overpowered in every single regard, broken from the ground up!
If you ever get the chance to evolve your chansey into a blissey, do not waste a single moment - Evolve it!