Portal

I had my doubts after hearing about how great Portal was, but I'm going to have to agree: Portal is awesome. The mind-bending portal gameplay is fun, but the script makes it shine. Whoever wrote the script is a genius. And whoever decided to hire the guy who wrote the script and the team that wrote the ending theme is a genius.

The plot in general reminds me a lot of Cube. The constant voice of GLaDOS also brings back fond memories of Shodan from System Shock, though she wasn't funny. I also read it's like the Island, so I added it to my Netflix queue.
The boss fight and ending credits really made everything worthwhile. Possibly the best ending I've ever seen, though games rarely have particularly memorable endings. I especially love the ending theme - Still Alive (no wonder- it's by Jonathan Coulton):

This was a triumph
I'm making a note here
HUGE SUCCESS!
It's hard to overstate
my satisfaction.
Aperture Science
We do what we must
Because we can
For the good of all of us
Except the ones who are dead

But there's no sense crying
over every mistake
You just keep on trying
'til you run out of cake
And the science gets done
And you make a neat gun
For the people who are still alive

Now that I get it, I've got to make a papercraft Weighted Companion Cube. As someone in the Gamefaqs forum said:

I'm going to make one.
Then I'm going to incinerate it.
It'll be just like old times!

I would never incinerate mine, though.

Portal: 5/5