Hmmm too many to count, over the years... I'll try to choose some less common examples.
Freespace 2: The Erinyes-Class heavy starfighter
This thing was a demon beast, given to you for the last couple of missions. Eight gun banks, twin secondaries, and you could load it up with UD-8 'Keyser's; you could pretty much guarantee a kill on every starfighter in the game within three volleys. Coupled with speed past a Herc-II and the defenses of a heavy bomber, it was close to illusion-breakingly hardcore.
Ghost Recon (original): British Army SA-80 Assault Rifle
Without meaning to start a GR flamewar, this weapon was absurdly accurate, low-recoil and had a fantastic scope. At medium range, made all other weapons obsolete and had strong enough long-range groupings to harass snipers.
Shogun - Total War: Warrior Fucking Monks
Seriously. Upgrade their armour and weaponry and they scaled like nobody's business. A few units of these could hold a centre against even no-dachi Samurai charges, and the only issue was expense and a theoretical vulnerability to arrows, which rarely seemed to amount to very much. If you had the economy, these were how you spent it.
Prototype: The Whip-hand
So you're an invincible bio-engineered killing machine with the ability to fly (glide, anyway...) scale tall buildings in seconds, alter your appearance at will, and cause bloody spikes to erupt from the earth? Well then YOU need a ridiculously versatile power that will allow you to a) decapitate whoever's within fifty feet of you b) charge up and spear single enemies and c) latch onto fucking helicopters in mid-air, drag you up to them so you can haul out the pilot and take over the damn thing...
Sins of a Solar Empire: TEC 'Insurgency' research option
Silly really. Dragging your empire to the conclusion of a civic tech tree in Sins will cause hostile groups of cruisers and frigates to spawn randomly in EVERY ENEMY SYSTEM, for no cost to you. Any system not actively defended by your enemy (or their allies) will have its' entire infrastructure blown to bits.