We've used lots of building systems. Lego, Erector, Zoob, Lincoln Logs, and many others.
Our latest one is called Keva Planks. They are so simple that at first glance you might dismiss it as a joke.
These things are unbelievable. They are sized with a perfect ratio of 1 x 3 x 15, meaning any way you build them they can align with each other at 90 degree angles. There are no accessories - everything you use to build is the same. This means no hunting for the right shape or color.
They claim to be "precision cut". I'm not sure what kind of cutting tools they are using, but everything lines up and balances perfectly. When building towers several feet off your surface, there is simply no movement at all in the structure. And we aren't taking the time to line them up well.
There is something about simplicity of this whole system that draws the eyes and imagination to it.
So far we have built a house, several towers, a helix, a bridge, and some coasters, which guide a ball down through a track.
We consider building with Keva Planks worthy of Math credit as the kids are constantly offsetting their structures by a given number of planks to create a slope for example.
I highly recommend this addictive building system.