It's not the hardest. But for a variety of reasons: craftsmanship, design, size and all around coolness, it is simply a masterpiece that will be in a museum one day.
I have some bad news: you cannot purchase this puzzle because they are all sold out. However, you can buy my article on it. Strangely, the August issue is on sale now. Don't ask me why. Suffice it to say, the September issue will soon replace it. So hurry.
GAMES Magazine can be purchased at Target, Barnes & Noble and any other place that sells a wide variety of magazines.
Here is Kagen's page: http://kagenschaefer.com/Lotus3.html