Isn't even suppose to be a toy yet. It's the unfinished wooden blocks I haven't used to make sets yet. The boys have legos, toys that go, block sets that are puzzles and sets that fit in wagons and oh so much other fun building related stuff. But the blocks that get dragged out the most are the unfinished ones that sit in a steel pot to the side of our homeschool supplies.
I guess is along the lines of a cardboard box being the most fun toy. That when you take away all the extra distractions of a design and color you are left with the best medium for imagination ;-)