For me, I like Apparatus
Apparatus tasks players with building machines to move marbles to a goal. It starts out almost insultingly easy, where the solution to one puzzle is setting a plank down over a gap, then becomes increasingly more difficult. The game's presentation is slick, and the puzzles are challenging. Multiple modes such as "sandbox" and the ability to play with other users' amazing creations as well as upload your own adds to the already high replay value of Apparatus. Rube Goldberg machine enthusiasts will be a kick out of building different contraptions in this game.
I love mind boggling games