The best designs, to my mind, are the ones that you can't possibly fully take in the first time you see them. Why? Because there are too many excellent details for your eyes and brain (and other senses, if you're seeing a thing in person and can touch, smell, and maybe also hear it) to fully process in that moment.
They're the designs that keep you coming back for more, occupying a space in your brain for longer than you're standing in front of them. The ones that keep you thinking about them for hours, days, and maybe years beyond that fleeting first impression.