1. The Kritzkrieg (Medic)

First released in 2008 during The Gold Rush update, the Kritzkrieg is an alternative to the Medi-Gun that has a faster-building Uber-Charge that grants critical hits instead of invulnerability. The pros and cons of trading invulnerability with deadliness is a tactical choice that makes the Kritzkrieg a must-have and a definite game-changer.

2. The Flare Gun (Pyro)

Released a few months after The Gold Rush update in The Pyro Update, the Flare Gun changed one of the Pyro's core limitations - being a short-range damage dealer. While not an incredibly powerful weapon; the weapon does crit when victims are already on fire, the Flare Gun gave the Pyro the ability to deliver fire-damage across long distances and become a major Sniper deterrent.

3. The Axtinguisher (Pyro)

Another Pyro Update entry, The Axtinguisher immediately became a must-have weapon - and is still a necessary tool for the Pyro. It deals critical damage to enemies who are on fire - 195 HP - enough to kill almost all classes in one blow. This meant that Pyros could set enemies on fire, and then quickly eliminate them with The Axtinguisher - the deadliest tactic for Pyros to-date. It's such a deadly weapon, and that makes The Axtinguisher a game-changer.

4. The Sandvich (Heavy)

The Heavy Update hit in the August of '08 and with it came The Sandvich. Less useful at the time, and better for making Om-Nom-Nom noises - The Sandvich today is an understandable trade-off for the Heavy's trusty shotgun. Bringing the Heavy to full health over the course of a taunt animation - The Sandvich can be risky - but sometimes no less risky than a slow, low-health Heavy looking for a Medic or Health Pack.

5. The Cloak and Dagger (Spy)

The Sniper vs. Spy Update of 2009 brought a lot of new weapons and tools for some of the more notorious classes of the game. The Cloak and Dagger, a new cloaking system for the Spy was one of the more significant changes to how the Spy could played. Instead of relying on picking up metal to stay cloaked - with the Cloak and Dagger a Spy simply had to stand still - and the cloak would last forever - even recharge. The Cloak and Dagger added new deadly Spy tactics and increased the success rate of existing tactics and is my final "game-changer".