The second best update of TF2 is the first one, The Medic Update. At the time, they just called it The Goldrush Update, but I think the medic part of it is more significant.
The new items for the medic, especially the Blutsauger and Ubersaw, really transform the class. The Blutsauger gives the medic a great escape method, allowing him to run backwards, firing wildly at his pursuers, hopefully draining enough health from his opponents to save himself. The Ubersaw, while used rarely, can be fantastic when it comes up. When a Spy crops up, medics instantly pull out their saws and turn into vicious hunting wolves.
Personally, I like to even sneak up on people in a melee and give them a couple taps, but that strategy isn't very consistent. Heavies don't like to turn their heads, but if they do, they melt you really quickly.
I'm also giving extra credit to the Medic Update for being the first of the major updates, thereby setting the template that would largely be followed by the later ones. (Thankfully, they didn't follow through with the impossible achievements, and having to get all of the achievements to get the items.)