For me the Pokemon that i think is over powered is that really annoying Miltank that Whitney has in Gen 2.

Rayquaza is banned from standard play, but if we're talking over powered, why not aegislash? that thing can wall the crap out of you and dish out powerful attacks, the only drawbacks are it's 60 base hp for walling and it's 60 base speed for attacking.

Mega Kangaskhan, it attacks twice and it's already powerful. One power up punch and it's much harder to take down cause of the attack increase. I am happy that it's banned from competitions.

I sometimes make sure I have a fighting pokemon in my team. I use to hate seeing mega kangaskhans when Pokémon X & Y first came out. This Pokémon was spammed like it was going to be the end of the world the next day.

Greninja, despite its defenses, can destroy almost anything thanks to movesets like this:

Ability: Protean
Item: Life Orb/Choice Specs
Nature: Hasty/Jolly

Surf/Hydro Pump/Scald
Ice Beam
Dark Pulse/Night Slash
Grass Knot/ U-turn

It can be considered to be Overpowered because of its ability, speed, special attack, and physical attack.

There are a lot of overpowered pokemon out there, excluding legends, here are some I think are very strong and why.

Aegislash-

Not only does it have stance change, but it also has a lot of good moves to utilize, like autotomize which will sharply boost that low speed stat. I have one with the moveset shadow claw, sacred sword, swords dance, and king's shield and it is a monster. 


Greninja- 

Greninja is really powerful because of its amazing speed and its great hidden ability protean which allows it to change into any type according to the move it just used. You can run a powerful special one, or a physical one if you want to surprise your opponent. 


Here are some other pokemon that are generally a bit overpowered, some of which are on the list because of their mega stone. 

Blaziken
Garchomp
Gengar
Kangaskhan
Lucario
Mawile


Then of course, there is Mewtwo who was already a very powerful pokemon, but now its stats are just off the charts with either mega evolution. 

Arceus. Base stat total of 720! The only thing that has a higher base stat is MEGA Mewtwo X and Y with a total of 780! If Arceus has a Mega Evolution, it would quite possibly hit the 800s. It's level up move-set is based on moves with a power of 90 or higher. And let's not forget about Arceus's signature, Judgement. 100 power, 100 accuracy, and the potential 16 PP makes this move devastating. To make it even more powerful, if you give Arceus one of its plates, Judgement will be changed to that type, giving Judgement a STAB bonus, making it even more powerful. Arceus also will only have one weakness in its normal type; fighting. Then again, it is the Pokémon god.

Th3 De4th Eag1e : hacked pokemon are the most overpowered thats why i use them a lot, but if you want natual stats mewtwo is super overpowered

For legit pokemon, try this:

Munchlax
Ability: Thick Fat
Nature: Careful
EV's: 252 HP; 252 Def; 4 Att
Item: Eviolite
Moves:
-Rest
-Curse
-Body slam/Return (max happiness)/Frustration (worst happiness)
-Crunch/Sleep Talk (depends on what you prefer)

This thing does wonders. If you run sleep talk instead of crunch, get rid of all the ghost (and perhaps rock/steel types) first, and then send in the beast. With the eviolite, you will eat up any special hits, especially fire and ice type moves. Aim to set up a curse ASAP, to boost your smaller defence and your attack. Once you get low, use rest and possibly sleep talk, and enjoy the fun. I have a little replay of what it can do in UU.

Skip to turn 12 to enjoy the fun

TornadoMudkip :


If we are including hacked pokemon, try this one: 

Shedinja
Ability: Sturdy
Nature: Jolly
EV's: 252 Speed is all you need.
Item: Lum berry
Moves:
-Agility
-Lock-on
-Sheer cold
-Recycle

Use it as a lead, it does wonders. Agility first, to outspeed pretty much anything. Then, if they toxic you or something, lum berry activates, and you use recycle next turn to get it back. Use lock-on/sheer cold to ko other pokes. Only gets owned by taunt and gastro acid.