The one in the background. Probably the most fun building a set from instructions we’ve had.
Overall though – the loose lego that we’ve collected over the years is the best value – projects on the go all the time. Most of them motorized like the Great Ball Contraptions, weaving looms, rope makers, cash registers, matchbox car firers, etc, etc.
(we also have mindstorms, so some of these things are programmed too).
The best thing is, I get all this under the pretence of it being for my boys! 🙂