Mike Hock, the first boss in a game called "Beating the One Eyed Snake". He has a mundane name but he actually gets harder the more battle againsts him and his length and girth are very overwhelming.

Economizer relic in Final Fantasy 6. Equip this accessory and watch as even the most powerful spell in the game now only costs 1 MP. Combine that with the relic that lets you cast two spells in one turn and....yeah.

Blood swords in Final Fantasy 2 are pretty ridiculous too, carving off a set percentage of an enemy's max health (not current health) with every swing...and you can dual-wield em.

This. It was a bit annoying and sad when every boss strategy suddenly became "spam quick + ultima" after that.

An awesome game, but severely unbalanced near the end.

Because it's an RPG first and foremost and not necessarily an FPS.
The system uses action points and once they're gone they either recover themselves slowly or you can use items to replenish them.
You aren't able to defeat a whole group of enemies with it. But if you have the right perks, it can make your life a bit easier. (obviousy it's not that necessary if you play it on the easiest setting).
With that said, you might be invulnerable for the time you choose your actions, but you aren't when the actions are being executed during the cinematic. You're also not guaranteed to hit either.
Really not that different from other RPGs where you can stop time and give orders (Dragon Age, Mass Effect for example) or RPGs that use a turn based system.

Summons in Golden Sun. You can take down almost every boss in the series with a round or two of summoning.

Well if you're smart enough (or just read up on the internet) there are lots of ways to overpower yourself in Skyrim.
Sneak, Enchantment, Alchemy etc.

Though if you play the Sneak + Bow combo without trying to break the game (grinding), I think it's not too bad as you don't even get that much experience towards the Sneak skill if you use the Bow. The Dagger though.... oh boy.

Sneak and Pickpocket level up fast together though, I managed to get Pickpocket to 100 and Sneak up to about 70 in only 3-4 hours of play. Granted, my Sneak was decent from the beginning. Pickpocket is ridiculously easy to level though.

I didn't really grind that much with my first character, the main thing with the Sneak + Bow is that even if it takes several hits to down an opponent; you'll remain hidden and still gain bonuses to every hit as long as you're undetected.

Obvious weakness of this is tight quarters and being forced into melee.

But, yeah, I agree fully agree especially with your first part, it is very easy to become overpowered in Skyrim, too easy in my opinion. When you have to limit yourself in the game so as not to become OP, the game design has taken a wrong turn somewhere.

Dragon Age Inquisition - Archer plus Artificer specialisation, full Focus.

Activate Hail of Arrows then basically use Leaping Shot over and over.

I took down a level 22 Dragon in 30 seconds (if that) at level 17.

Ultima Weapon in Final Fantasy 7, it forever broke my expectations of damage I had at the time in RPGs and because I had several HP pluses equiped at the time and then discoved 4xCut, it just killed the competition outright. Cant deny it was fun though :D

For anyone interested in RPGs btw, Konami is having daily Suikoden stream to measure interest and potentially save the Suikoden series by showing that people still care about it. If enough people join in, discuss it and buy the digital releases they may release the games on Steam, make sequals etc.

So please join in at http://www.twitch.tv/konami 3:30 PM Pacific time every weekday and discuss the series or just discover it if you're curious.

hp regen, I remember in Modern warfare, you could get shot with over a million bullets through a level and still be alive like nothing happened. Imagine soldiers doing that today.