The Rubik's Cube... the bane of many men. Its difficulty is extreme, especially if you don't look up methods or formulas ont he internet. A man spent 25 years doing it... For the low price of ~ $15 - $20, this thing can keep you entertained for hours and even years. Sometimes you just want to throw it out of a ten story window and hope to god that it gets flattened by a truck, but when you complete it, and then learn how to do it faster and faster, it is more rewarding than any other game. This puzzle allows you to think logically, and also measure your parameters of frustration until you ragequit :P. I remember when my entire high school grade bought these, and each lunchtime was just sitting down with these newfangled contraptions and trying to solve them... and there would always be that one kid who could solve it super fast, and everyone gathers around him, trying to leech of his mad skills.
This is probably, no DEFINITELY the best ~$15 - $20 dollars you will spend on a piece of plastic. Also, always remember to lube up your cube with some vaseline oil :D.