I think the reason the majority of the people defending Yahtzee's trustworthiness are doing so is because while he may be biased, he's at least open about it. We generally have a pretty good idea going in of whether he's going to like a game or not based on the genre and content. He also has high standards, so people know that when he says a game is good, it's probably pretty damn good.
I have found that nearly all of his reviews point out the flaws quite well, and as he's said, he's a critic, it's his job to criticize. Despite all the shit he rakes up, he actually has liked a lot of the games he's reviewed. Obviously he makes a lot of use of hyperbole, but that's part of what makes it funny; I just make sure to take the criticisms with a cup of salt, as they probably aren't as annoying as he makes them out to be. I also tend to put a lot more stock in story than he does, regardless of how cliche it is, so I know that "too much story" for him is often "cool, lots of story" for me.
Anyway, to answer the original topic, the reviewer I trust the most is myself playing the demo. Yahtzee hated Burnout: Paradise, but after playing the demo I found the visceral crashes to much to pass up, and after buying it found that I really enjoyed exploring and learning the ins and outs of Paradise City. After playing the Tomb Raider: Underworld demo, I think that's on my list too, at least to rent. Long story short: there's a demo for nearly every game nowadays, and it's usually a good indicator.
Other than that, I've come to trust GameTrailers, for a few reasons:
1. They balance the good with the bad. They actually often tend to bring up a lot of the same criticisms as Yahtzee.
2. They take the perspective of a fan of the series/fan of the genre. They know that people who hated the first two Halo games (read: most people here) aren't going to buy Halo 3, so they focus on what it brought to the table that the second one didn't, which is the selling point for fans of the series. The same goes for fighting games, or JRPGs, both of which we know Yahtzee is inherently biased against.
3. They review all the content. Staying on the Halo 3 example, I don't think Yahtzee realized that by the time of the third installment, the main draw of the series, for most fans, was the multiplayer. Sure, they wanted to "finish the fight," but most people wanted great multiplayer from the title more than anything else.
4. They rarely, if ever, actually contradict Yahtzee's points, and the consistency adds to the notion that they aren't getting free money for giving the game a good score.