I had been going to say it was also in the running for his simplest game but the super tiny dice game Katego holds that title pretty tightly. Maybe the Flinke Pinke family is his simplest game that still has solid replay value and interesting decisions :P
The game consist of five suits of cards, ranked zero through five, and sense of chips which match each suit. Most versions of the game have colors for the suits. You play by dealing out the cards. On your turn, you play a card and you take a chip which doesn't have to match the suit of the card. The game or around ends when the sixth card of a suit is played.
At that point, each chip is worth as many points as the last card played in its suit. So, for instance, a blue chip could be worth as much as five points or as little as zero. Whoever has the most points, that would be the winner.
Finke Pinke et al has hand management, bluffing and timing. You aren't just playing your hand, you're playing the other players. Both the cards you play and the chips you take can speak volumes. Sometimes you're working with another player. Sometimes you are working to fold them.
Don't get me wrong. This is not a super deep game, hiding intricate decisions beneath it simple components. But, at the same time, it offers some interesting and even tough choices during its short playing time. I have even heard some people call it the simplest stock market game ever, although I cannot consider it the stock market game by any stretch of my imagination. (No buying and selling as part of the value adjustment)
I own both ends of the spectrum of this game. Loco with flimsy cards and plain chips (that don't even fit properly the box :D) and Quandary that has a board and all the other pieces are Bakelite. (Really, its over the top) Oh, and I have Thor, which ends action cards. My copies is in German :P
Finke Pinke et al is a tried and true example of how simple rules and short playing time can still lead to a full and satisfying game experience. Don't give me wrong, there's plenty of examples of that. But this game has been doing it since 1994.