Stacking Cups
Toys 'R' Us, 12 months +

(This is a generic toy, branded by the seller. Our example is Toys 'R' Us branded, however as this photo illustrates it can carry other branding.)
This is a collection of ten stackable, nestable cups. Each cup is a unique size and they stack or nest in numerical order. Each cup has a number on the base along with an illustration (ie number 6, and six bees). The set comes in a bucket which can be used as an eleventh 'cup'. The bucket has a lid with four sort-o-shapes cut out of it; four sortable shapes are also provided.
The stacking cups are hands-down the best toy we ever bought. It is absolutely his favorite indoor toy, and possibly his favorite toy overall. Alex is constantly fascinated by stacking and nesting the cups. This can keep him focused on the task for up to 20 minutes, by far the longest he has been able to stay on a single task. It can also be a social game, where an adult helps with the stacking and nesting, keeping cups from rolling away when discarded.