There are many good ones, but it is hard for me to give a short answer because there are so many wildly different genres and styles, also games that take 2 hours versus games that take dozens of hours to complete.
For short games, you cannot go too wrong browsing some of the annual competition winners: The Impossible Bottle, Lost Pig, etc…
Hadean Lands is a massive, intricate, and difficult puzzly game, well worth the five bucks if one is into that sort of thing.
Counterfeit Monkey is a wordplay puzzle game I have mentioned before.