Of all the years I've been with Pokemon, the most hated Pokemon seems to be Zubat. Not that they're weak or anything, it's because they're annoying. You encounter them literally every cave you come by. This comes from the opinion of those who mainly play the games, and not competitively.
Now, competitively speaking, a lot of Pokemon receives hate for being "overrated", "OP", so generalizing on just one Pokemon would be harder than it seems.