Lucario in Pokemon B/W 2. You can get a Rilou near the start of the game, and his movepool can let him sweep the entire E4.
Fallout New Vegas - The laser rifle, fully modded. And with how dumb the AI is for some creatures, mines (Deathclaw, Im lookin at you.)
Phantasy Star Portable 2 - Twin Sabers with the Infinite Storm photon art.
Gundam VS Gundam Next+ - Kamille's Gundam MK II. It is a 1k cost unit - Lowest cost in the game. And it can easily destroy all other 1k units, most 2k's, and a couple of the 3k units. Oh and, if 'dies' when it's no longer able to respawn? Well, it comes right back - minus a arm - to fight some more.
Heavy Gear II - With it's customization system, you can make any of the mech's OP. Sheer numbers though, giving a mech a Panzerfaust and maxing the ammo will boost its threat ranking right through the roof. The customization system works both ways though - While you can make any of the mechs OP, you can also completely screw them. (Light, close range only weapons? Check. Only a shot worth of ammo? Check. Low armor, sensors, speed, and manuverability? Check. Stripped armor, bad sensors, a screwed fire control system, ammo containment that frequently blows up for no reason, and legs so bad that they will break on a dime? Check. Yeah, with the system you can make a mech that will go boom from a single shot.)
Front Mission Evolved - Some of the unlocked at the beginning of the game DLC weapons are as strong as the original end game weapons. That said though, anything that you can mount dual gattling cannons to will be op.