Just post the most important version update and tell why you think that.
I think v9 was the most important as that is when the eventing system came along enabling addons such as JVS and allowing people to make fancy contraptions, and I don't know where I'd be without it.
Here are some of the key features from some past updates that I can remember (refresh your memory)
v9: Events
v11: Brick Physics
v12: Several small changes
v14: No more port forwarding
v15: High Speed Add-on download system, you can now build inside water bricks
v18 : Downloaded files are now stored in a cache file rather than cluttering up the add-ons folder
v20: Launcher update
v21: Shaders, one adjustable slate map
(I'll add more versions to the list as people suggest them.