It wasn't the first game to utilize the concept of an auto-moving unit, but it absolutely popularized, and for good reason. A ton of levels requiring you to use the various tools at your disposal made this the most robust example of the concept when it came out, and the game did a great job on building your skills and ideas about how to problem solve as you progressed through the levels.

Hours upon hours of ingenious puzzles to solve. This is the first game I remember playing.