Which site should I have in my bookmarks to read game reviews before buying a game?
1 site per answer, please.
I use Metacritic . http://www.metacritic.com/games/
Always has nice video's with lots of ingame footage.
They always discuss the design, story, gameplay and the graphics. Each category get's graded and lead to an overall impression. http://www.gametrailers.com/game-reviews
Zero Punctuation  has surprisingly entertaining video reviews. http://www.escapistmagazine.com/videos/view/zero-punctuation
GiantBomb , formed by ex guys from GameSpot
The most interesting video section there is "Quick Look", where they just play new and unreleased games for half an hour. http://www.giantbomb.com/videos/
For PC gaming and general silliness: Rock, Paper, Shotgun 
They have excellent coverage of the indie (and mod) scene.
Wikipedia  says:
The site's scope is broad; it reports on upcoming major releases and independent esoterica, and includes reviews, previews, features and interviews related to PC gaming and the PC gaming industry.
Avoiding the style of traditional reviews, a more subjective style is preferred with either a "Wot I Think" write up, focusing on the author's personal experience with a game, or an "RPS Verdict" where all four writers offer their opinions of a game culminating with a "thumbs up", "abstain" or "thumbs down" from each of them.
I'm really surprised no one has mentioned Kotaku  (linked to one of their reviews). I really like their like/dislike style. Even if you don't agree with the specific reviewer, they make a point of explaining just why they feel like they feel about the game.
Mind you, reviews are not Kotaku's core business. http://kotaku.com/5581030/singularity-review-wibbly+wobbly-timey+wimey
I usually read the reader reviews on GameFAQs . You may have to wade through a lot of poorly written reviews until you start recognizing the names of the good contributors, but at least they offer a wide variety of opinions from individuals. http://gamefaqs.com
Find yourself a blog with similar tastes in games as you.
Rating systems are outdated in my opinion, and frankly after things like the business with GameSpot and Jeff Gerstmann, I'm reluctant to trust the big sites to write an unbiased review. And now there's post-release review embargoes.
Nobody else seems to have mentioned it thus far although it's more of a gaming news site than a review site (though they do review stuff!), for me it's the best mix between news/reviews in a very easy to read presentation.
www.ign.com  http://www.ign.com
My personal favourite is destructoid. http://www.destructoid.com/
Great site with a great community. If they don't do a formal review, you're likely to be able to find some detailed opinions in their forums.
I quite agree with Brian Ortiz , and would recommend to stay away from review aggregators, and all this recent stuff.
Find a place where the reviewers have tastes/humor/style which suit you. The mark in the end is not that important.
I would recommend Rock Paper Shotgun . They don't review all games, only a few ones, and even then, it's not an actual review, it's a "Wot I think" . It's a writing about the game, without an actual mark at the end. And all this in smart, British humour. http://gaming.stackexchange.com/questions/416/good-websites-for-game-reviews/655#655
GoozerNation.com  is a small gaming site that does reviews of the popular games for the month. It is a small site with a variety of gamers writing for it. http://GoozerNation.com
GameSpot  is my favorite. http://www.gamespot.com/
Good Game , more of a TV show format, but still some good reviews out of them. Plus, the reviewers are not too hard on the eye
Also, Good Game SP  for the younger crowd. http://www.abc.net.au/tv/goodgame/
I prefer GameRankings.com  to Metacritic, personally. They operate the same as Metacritic (averaged review scores), but they're focused only on games. Plus, I think Metacritic's site is uglier (personal preference, I know).
I never use the average score as a basis to buy or not. What I usually do is read 1-2 of the higher reviews, 1-2 of the lower, and maybe 1 that's average-ish. You'll find that sometimes a particular reviewer rates a game high or low because of a particular aspect of the game, which may not matter much to you. (For example, to use an old game: I don't care if Metroid Prime 2's multiplayer sucks, because I have no intention of playing it. I only want to know how the single-player part compares to the first. If a low review score seems to be based on sucky multiplayer, that doesn't matter to me.) http://www.gamerankings.com/
If you listen to any gaming podcasts (like 4guys1up , Joystiq podcast , GiantBombcast ), you can often hear longer discussions of games and they'll go into detail about things they did and didn't like. Try to find one where the podcasters seem to have similar tastes to your own. I didn't think much about playing Silent Hill: Shattered Memories until the Joystiq Podcast guys talked about how much they loved it. I'm playing it now and finding I really do like it. http://www.1up.com/do/minisite?cId=3176640
IGN is a good site because they have a video review for all popular releases with gameplay footage and an explanation of each category (single player, multiplayer, graphics, controls, sound/music, etc)
I'm a big fan of xbox360achievements.org . As well as the forums there and their usual guides/achievement lists etc, they also do game reviews of most decent sized releases. I like the fact that they give breakdown scores for different categories such as Graphics, Sound, Longevity, Replay Value etc, rather than just an overall score. http://www.xbox360achievements.org/
Personally, I use Bit-Tech . They are not a games review site, though gaming is within their remit, but I'm a regular there and I know and trust the staff. They don't review all games, but they cover all the major releases and also bring my attention to indie/lesss high-profile games.
They're not one of the many gaming sites that accept bungs for positive reviews, and they are needing to be mindful of advertising revenues, so if a game stinks they will say it stinks. If a game gets a good review, it is likely because they think it deserves it.
I avoid sites IGN and Gamespot... http://www.bit-tech.net/gaming/
For PC reviews I use Rock, Paper, Shotgun. But that has been mentioned elsewhere in this topic.
For non-PC reviews, I use Eurogamer . http://www.eurogamer.net/
Action Button 
While they do not update as regularly as the commercial sites, the quality and enjoyable style of the reviews more than make up for it. http://www.actionbutton.net
I personally like 1UP's reviews . My favorite genre is is RPG and they have a good RPG staff with a great RPG blog (however, their podcast is just plain annoying). 1UP is the successor to the old Official Playstation Magazine, but is now owned by UGO Entertainment and intended to be a community-focused site. http://www.1up.com
For me I always check IGN and Xplay (G4tv) Before buying any game. I find that I agree with them 95% of the time and that they show plenty of gameplay to support claims.
The only sites I can say are definately not good for reviews are Zero punctuation (EscapistMagazine) and Gametrailers.
If you listen to Zero Punctuation you will never buy anything, he hates even the best of games, sometimes promoting sub-standard ones.
Gametrailers a while back made a comparison video of a game on Xbox and PS3, however on the ps3 one at one point a quicktime event button showed up that clearly showed it was actually just showing another Xbox, this to me either suggests Bious or Lazyness. As a result I know longer trust them.
At least there.. 'All sorts of people post reviews' not just hardcore gamers. http://www.amazon.com
I prefer to videogamer.com . http://www.videogamer.com
I hang out on I Love Games a lot. Good, smart group of guys with varied tastes and real solid knowledge. http://www.ilxor.com/ILX/NewAnswersControllerServlet?boardid=67