It's cool when games are fine tuned, well balanced, and the difficulty feels spot on. On the other hand, sometimes there is nothing better than an OP weapon that makes you feel like you completely owned the game. A classic example might be the pistol in Halo: CE.
For me, the one that comes to mind is the Scout unit in the first Valkyria Chronicles. You can run around all over the place cuz its stamina is crazy. The rifle's damage output really isn't that bad and it has a pretty awesome range. It's simply miles better than all the other units. Deploying a bunch of them makes you feel like a filthy cheat but also like an elite squad of badasses.
What OP thing in a game gives you the most satisfaction?