4D is my favorite puzzle made by George Miller of puzzlepalace.com. It had just the right amount of difficulty for me with a fascinating premise: Solve four mazes at the same time. Each of the individual mazes is connected to two other mazes in different dimensions. I could spend an hour just being impressed with the mechanics of how the pieces of this puzzle move and are connected and related to each other. You can't visualize it from the picture.

When a co-worker dropped the puzzle and one of the acrylic pegs broke, George Miller sent me a replacement (and an extra spare) at no cost. Fantastic quality puzzles and fantastic service.