Portal is an amazing video-game experience unlike any other.
You play as Chell as you run through various "test chambers" trying to locate the exit and get to the next one. Like Gordon Freeman, the protagonist of Half Life, Chell never says a word, this can be rationalized with the fact that Chell never meets another person. The only dialogue comes from Ellen McLain who gives voice to GLaDOS (Genetic Lifeform and Disk Operating System) and later the most adorable automated turrets you will ever find. Ever! GLaDOS starts out with funny lines and then all her lines turn into solid gold and I found myself hysterically laughing in my chair with fear of falling over or losing bladder control. The other only thing you come across is your Faithful Companion Cube which will help you later in the game.
The only flaw that you can rationalize with Portal is that it isn't very long (I finished it in 3-4 hours) and for, probably, 99% of all games that would be considered bad but in Portals case because if you tried to put in 10-15 hours of gameplay you would just postpone the inevitable and I'm afraid the game could get a little repetitive. The length of the game fits perfectly with the game and makes it a game that you can pick up and finish in an evening so you can hear all the great comments from GLaDOS.
When people ask me now what I think is the best game ever made I'll have a clear answer: Portal. A strong and well deserved 10.