If you want a challenging strategy game for 2 players, then Pentago is a great choice. 

It's an exciting game which has simple rules but offers an infinite number of combinations. 

The object of the game is to create a row of five marbles in any direction. But on each turn you must also twist one of the four game blocks as part of your move. This causes the game board to constantly change, making for a perpetually challenging game experience, both for beginners and masters alike. 

Because you can rotate quadrants, your mind has to plan out moves in so many different directions.   And just when you think you're a move away from having 5 marbles in a row, your opponent rotates a quadrant and everything changes.

Pentago won the Mensa Select Award in 2006, as well as many other game design awards.   It's recommended for ages 8 and up.