My favourite for the last few years has been Stormworks. It lets you build a lot of various vehicles with a lot of creative freedom. You can use out of the box controls, get a bit more advanced with the in-game microcontroller editor or go even further with lua scripting. I dove in blind and love it. Then there's transport, logistics, rescue, research etc. missions to complete.

Others I have played before are Scrap Mechanic and Besiege which are a lot more lightweight and easy to get into, but with less advanced building possibilities.