If puzzle games are your thing, then Portal is a must-play. Both games are exceptional, though the first was groundbreaking. From Valve's Orange Box, which most people initially bought wanting to play Half-Life 2, Portal was an unexpected gem which quickly gained fame, and for good reason.
In terms of problem solving, Portal pushed the limits with how you could use a 3D game to figure out puzzles, improving spatial awareness and attention to small details. It implemented challenges which tested the player's intellect, making it a rare mainstream success which encourages brains over brawn or even a player's mechanical ability.
While this game has little replay value for the main story, it has time trials for the vaults which can lead to hours of additional enjoyment; it is a definite must-play. In terms of thinking outside of the box, Portal forces you to see the world differently, and has been an inspiration for many other games which put wits before hits.