But more in the spirit of the question, I’d say Popoids and Construx. A few days ago I looked up the Universal Connector Kit to see if any more pieces had been added (there haven’t been) and got to thinking about my favorite building blocks from when I was a kid. Popoids were these plastic tubes, like the bendy bit of a straw, that attached to plastic blocks. And Construx were kind of like plastic construction beams that could be built up into three dimensions.
Why? Honestly, just for nostalgia’s sake.