One of the best puzzle games for sure, the portal mechanic was pretty innovative for it's time, and even though the game is nearly 15 years old, I think it holds up pretty well.

It's a short game, around 2-4 hours long, but the levels are well crafted and some of them will probably take you a while to figure them out, and although the game has close to no story, it has a memorable villain who you'll probably come to like.