I was just thinking about this and you know what? It's a toss up between 1.3 and 1.11.

The 1.3 update brought Simpleplanes to steam, ushering in a new era of playing. The 1.3 era broght us racing, weapons, and combat, thus changing the game. Pc users got two new islands to explore as well.

Then came the 1.4 era of course with it's wheels, car engine, and heli parts.

But 1.11 changed the game. Interior cockpit parts. Glass. After 1.11 there really is no excuse for a plane not to have a full functioning cockpit.