Most RPG's allow the PC to become overpowered I'd say. Just a few examples:
Fallout New Vegas: There are dozens of ways. Any of the obviously lethal weapons of course (Brush Gun, Anti-Material rifle, gauss rifle, etc) but most surprising is a handgun. Without cheating, one can get the unique weapon "Light Shining in the Darkness" - essentially a custom model 1911 commander. Natural damage of the weapon is sufficient to punch through the armor of anything in the game with ease and using specialty rounds can really increase the hurt. With a good build, you run into something surprising: it is one of the most lethal weapons in the game. It has an absurdly low AP cost to fire - without trying terribly hard (just the action boy/girl perks, a reasonable investment in Agility at the start and an implant) you can fire the weapon 6 times and reload and fire again in a single vats sequence. Beyond the startlingly high base damage (easily possible to do 70 or more damage in a shot) and rate of fire there is something even more incredible: the bonus to crit chance. Combined with a proper build and other pieces of equipment (any light armor especially if it increases crit chance, NCR ranger beret, high luck by whatever means, finesse perk, etc), the weapon crits with staggering reliability ((10% from luck, +5 for beret, +5 for finesse, +5 for light touch) * 2 for the gun itself = 50%! Drugs and magazines can push it further still). What you have is the ultimate weapon for hardcore mode. It is light, it is best used with light armor, and it does staggering damage. It reliably deals damage on par with a Ranger Sequoia but fires three times faster and has a shorter reload sequence all with easier to make and cheaper to buy ammunition. What's more, it's DPS is bested only by a handful of weapons in the vanilla game natively and the effective ability to deal damage quickly to anything is only beaten by the largest and most absurd weapons.
In short: with the proper build, the handgun is more generally powerful and useful than any number of rifles, shotguns and explosive weapons and is only beaten in terms of pure damage by weapons with terrible flaws (silly heavy and expensive ammunition for example, or sheer danger of using it at the average ranges you'll fight at in fallout).
Skyrim: There are lots of ways to become overpowered. Sure you can make a hammer that does 1k damage a swing or a bow that unleashes 350 damage cruise missiles. These are all stupidly powerful. But there is one item that makes the rest of them redundant: Chillrend. As a leveled item, your mileage may vary and certainly the enchantment on the weapon is weaker than what a player could apply (at best 30 frost damage and 2 seconds of paralysis compared to the easily attainable 60 frost damage and 8 seconds of paralysis). This is trivial when compared to the fact that if you pick up the sword at level 46 or higher, it does more damage than even the Daedric sword - by a fair amount. Combine that with the various tricks for breaking the game via absurd damage and you get a sword that does more than 450 physical damage on a swing, recovers quickly, and offers options for defense or even greater assault that are simply staggering. Few things in the game survive a single normal attack. City guards crumple in a power attack. Yes, there are things that do more, but more is never necessary.
