Portal 2

Welcome back, test subject. It's been a while.
Portal was one of those games that took the world by storm. If you're reading this, you're pretty much guaranteed to know what it is, but for the benefit of those who don't: Portal is an action game set in an abandoned scientific research facility. The principal mechanic of the game is the Aperture Science Handheld Portal Device, a 'gun' that creates 2 linked holes in space-time. This portal device is used to great effect, creating head-scratching physics puzzles - and yes, Valve show off their physics engine once again. What no-one expected, however, was for the game to be funny. Dark, deadpan humour from GLaDOS, the psychotic AI in control of the facility. Portal created one of, if not the most well known video game memes of all time, 'The cake is a lie.' (horrifically, the line is actually misused most of the time - it wasn't supposed to be a silly random line, it's more to give the player an understanding of GLaDOS' grasp of the human psyche, that cake is enough of an incentive to make the subjects continue through the frankly horrifying test chambers. In the end, you break free of her control, work your way through the interior of Aperture Science (the facility), and find GLaDOS herself.
You burn her.
Following up to what is perceived by many as one of the best puzzle games of all time, then, is no easy feat. But then, this is Valve, a company who cut their teeth on the universally acclaimed Half-Life and Counter Strike. They can do follow ups pretty well, considering the universal acclaim that Half-Life 2 receives. They do have a little trouble with the number three, however - see the MIA HL2: Episode 3 and the lack of Left 4 Dead 3 as evidence. So, when Portal 2 was announced at the end of a week long Alternate Reality Game last year, people were understandably excited. The concept sounded excellent - GLaDOS is back, Aperture Science is in disrepair, there are new characters in the shape of personality cores that will join you along the way
Fast forward a year, 2 weeks before Portal 2 is released. It's April 1st, and 13 indie games are updated with the message #happyPotatoFoolsDay. Something's afoot. Thus followed another ARG that lasted for 18 days, involving all forms of media - the 13 games themselves, Youtube videos, people visiting the headquarters of one of the indie studios and the community realises - this is to do with Portal 2. We're treated to concept art from the game and an extra level in each of the indie games directly relating to Aperture Science and GLaDOS. Valve had kicked their hype machine into the highest gear, and we loved it. The ARG culminated in Portal 2 releasing about 10 hours early.
This may all seem irrelevant, but it's important that we realise how big this game is for Valve. Gabe Newell, Valve's CEO and founder, was quoted as saying 'Portal 2 will be our best game yet.' There was a lot of hype surrounding this game.
Fortunately, it lives up to it. Chell, the player character from the first game, returns after she is rudely awoken by Wheatley (voiced by Stephen Merchant, of Extras fame) to find that Aperture Science is being destroyed. What follows is a 'train ride' (somewhat characteristic of Valve games) through the facility while you watch the room you're in crumble around you. At the end of this, you reach the Relaxtion vault from the first game, find the portal gun, and complete a few chambers that may look a little familiar, if a bit overrun with plants. You meet up with Wheatley again, who takes you on a quick behind-the-scenes tour to the nearest 'escape pod' - which just so happens to be in GLaDOS' chamber. I'm sure you can imagine what happens next.
The game is split into 3 main puzzle sections, each controlled by a different character - the first being GLaDOS, naturally. In between these sections, there are the 'story' areas, where the puzzle solving takes a bit of a back seat and you are instead introduced to the meat of the story. This is mainly due to the lack of dialogue in the test chambers - a decision made by Valve in the first game to ensure the player was fully concentrating on the puzzles and wasn't being distracted. The story is genuinely great, with a couple of great twists, and an ending which will go down in the history books.
Puzzles are solid, albeit a little on the easy side - there is no flinging from the first game, and at least until the final third, the puzzles are pretty easy. The last set of puzzles are far more challenging, and seem to be the direction Valve wanted to take all along - unfortunately, their testing team had other ideas. The game suffers a little from this 'design by committee' aspect - playing through again with the developer commentary enabled, you hear that Valve had some excellent puzzle ideas, but bad playtester feedback held them back - things that sounded truly great were cut because they were too difficult. A new testing element is introduced often, and combined with the previous elements in interesting ways. The parts I enjoyed most were the gel puzzles, although I seem to be alone here - many others have said that these were the puzzles they found most boring.
But the most important aspect of Portal was always the acting. Without it, we wouldn't have had the excellently delivered humour of the original. Thankfully, the same applies here. Ellen McClain is outstanding as GLaDOS, who is now a more offensive character after you so rudely killed her in the first game. J. J. Abrams is Cave Johnson, the founder of Aperture Science - he has some brilliant lines that will genuinely make you laugh out loud. He isn't around for long, though, and the game suffers slightly for it. The best character, though, is Wheatley. A scatterbrained personality core, he is the comic relief. He's voiced by Stephen Merchant, a man who can improvise brilliantly, and is absolutely hilarious. I often found myself standing around instead of continuing the game in the hope of making Wheatley say something else. Merchant fits the role perfectly and his lines never let up in their quality.
So that's the single player, but for Portal 2, Valve added co-op.
Co-op is far less story based. You play one of two robots who are being tested by GLaDOS, and it's immediately clear where all those clever puzzles from the single player went. The puzzles in co-op are hard, making use of all 4 portals available to you, as well as you and your partner's combined brainpower. The final puzzle is one that took me and my partner over 20 minutes, but when we got it, by god was it worth it. It takes all the testing elements from the entire game and rolls them into one multi-tiered puzzle. Brilliant stuff.
Portal 2 is pretty short. The single player took me 10 hours while looking for easter eggs and listening for extra dialogue. Co-op took about 4 hours. Getting all the single player achievements and doing another run with developer commentary added a further 5 hours to that total. I feel that 19 hours for £29 isn't bad (discounting that I also bought the Potato sack from the ARG, which was £20). But really, we shouldn't  be judging games based on length. We should be judging them based on quality and the experience. And on this, Portal 2 excels - it's certainly Valve's best game, and soared to the top of my personal list of the best games of all time. You have no excuse not to own this game yet. Buy it.

P.S. It also has hats, if you're a TF2 type.