When it comes to everything from Premise to Puzzles, Portal takes the cake.
Overall the game's difficulty is pretty challenging without becoming impossible. We're no Gordon Freeman, but we were able to figure out some of the more complex puzzles. Grasping the concept of flinging takes a little while to wrap your head around.
The sound is wonderful. It's mostly just the environmental sounds of the background. Think there was a little techno put in there like HL2 did during some firefights. Graphics are top notch. That should go without saying, but it deserves repeating.
There isn't much of a downside with this game. I suppose replayablity will become an issue after completing the game, the achievements, and the advanced maps. It's a linear game in terms of story. Some puzzles may have multiple ways of solving them, but it doesn't effect the game. It's a short game, but that's not necessarily bad. If it had went any longer with just the story provided it might feel repetitive. There aren't a lot of characters in the game, if there were more it would have only gotten better. Save it for a sequel hopefully.
Yeah... so... great game, great gun, what more do you need?