PokÈmon Mystery Dungeon 2: Spacial Rend, Charge+Discharge combo - These two allow you to slaughter hordes of enemies (if they are in the same room) very, very quickly.
Total Annihilation: ARM Annihilator - expensive/slow to build, slow rate of fire, sucks down 2,000 energy each shot and the barrel is slow to track. But it has great range, does extreme damage (kills almost anything in one blow) and the projectile moves extremely fast
Total Annihilation: ARM Vulcan - VERY expensive/slow to build, sucks down 5,000+ energy at full throttle and the barrel moves very slowly. But it has great range, a very fast rate of fire and the shells do major damage. If you get it built within range of an enemy base, you can completely level it in short order.
Plants Vs. Zombies - Cob Cannon. Requires 700 sun but each missile can instantly kill several zombies. Only problem is that it takes a while to reload.