The Javelin from MW2 is the greatest thing ever. Use it right and you can get tons of kills with it.
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The AT-420 Sentinel from The World Is Not Enough(N64). It's a big-ass breadbox with four -blam!-ing barrels, what's not to like? You can guide the rockets with a laser or just fire everything at once, instantly vaporizing the poor mother-blam!-er standing in your way.