Viability ratings wouldn't make a lot of sense, because most Pokemon's viability depends on the mode they're used in and what team they're on, and they often won't work particularly well on other teams. Durant is complete trash outside of Singles, where it's godly. Togekiss really isn't all that effective in Singles, but I got a streak of 543 with it by capitalizing on its typing, bulk, and access to Yawn and Safeguard to support Mega Kangaskhan. But even overwhelmingly good Pokemon like Mega Kangaskhan, Aegislash, and Greninja can be absolutely demolished before battle 50 if they don't have the right support and if they run into the wrong opponents.