Before I get to my list, I think I should straighten some things out. Overrated means that the pokemon doesn't deserve the praise it gets because people treat it like it's a lot better than it actually is. A pokemon isn't overrated simply because you don't like it. Here's what I mean:
Gen 1 - Charizard, I'll go with the group and say Charizard for plenty of reasons that people before me have stated. It looks cool though.
Gen 2 - Scizor, I'll admit, Scizor is an incredibly powerful pokemon, but I've never really had a problem disposing of it. Just about any physical wall with a fire type move can easily counter it. Even Tangrowth, with a type disadvantage and mine has HP ice instead of fire, has almost no problem taking it down if focus blast hits enough.
Gen 3 - Breloom, let me get one thing straight, I love Breloom. It's a powerful pokemon and a great addition to my grass team, but it's completely shut down by ghosts, dragons, or physical walls. It has a fierce reputation that is well deserved for unprepared teams, but it is a threat that can be easily countered.
Gen 4 - Honchkrow, it's a good attacker, but its frail defenses means that even a resisted attack will do a good amount of damage. It's not even that fast, so a good number of pokemon can counter it, even if it's weak to brave birds.
Gen 5 - Terrakoin, this legendary finds its happy little home on several OU teams. However, this pokemon is immediately shut down by priority. It's typing makes it so that one well-placed mach punch can quickly end its reign of terror. It might be just me, but I never had a problem taking it down.