#3. Blue Shell (Mario Cart Series) - Blue Shells automatically lock-on to whoever happens to be in 1st place, which is hugely useful as long as you aren't already winning. Certain games in the series have made Blue Shells dodgeable, but it takes skill. Launching a Blue Shell can completely change the outcome of any race.
#2. Tactical Nuke (Call of Duty: Modern Warfare 2) - Tactical Nukes in MW2 proved to be quite controversial. When the killstreak is used all players' characters are killed, the match ends, and the team that used the Nuke wins. Once someone has one, there is absolutely nothing that can be done to stop it. The only think keeping this weapon from being #1 on this list is that fact that it is incredibly difficult to get. It takes a whole lot of skill and luck to amass the 25 consecutive kills required to get this killstreak (and of course you had to unlock it and select it, too). Unlock the others on this list, I have never acquired a Tactical Nuke, but I have been on the receiving end of one.
#1. Master Ball (Pokemon Series) - You may not consider the Master Ball (or any Poke Ball) to be a weapon, but I do. You use it to capture a "pocket monster" that you then use to fight on your behalf; sounds like a weapon to me. What makes the Master Ball great is that it automatically captures whatever Pokemon you used it against. In the original Pokemon Gameboy games (Red and Blue), it was the only way to catch the legendary Pokemon, Mewtwo. Even though the Master Ball is invincible, you only get one. If you don't hold onto it (and save it for Mewtwo), then all you can do is waste it. The Master Ball has taught millions of gamers an important video game (and life?) lesson: hold onto what is most important to you.