It is probably a close tie between Rotten Tomatoes and NetFlix. RT usually does a good job, but sometimes way off. Netflix is harder to judge because it only allows four stars. More granularity would help.
IMDB is the worst of the 3, often giving great ratings to terrible movies and bad ratings to movies I like.
How about you?
I trust IMDB when it comes to older movies; for newer ones it is virtually worthless for at least a month (often several months) after the release date because of all the idiot votes (1’s and 10’s) a new movie invariably gets, and even then there have been several recent films whose IMDB ratings I still don’t trust, at all (Snakes on a Plane anyone?). RT is better for new releases, but I dislike the binary rating system (thumbs up…err fresh or rotten). I don’t pay attention to Netflix’s ratings even though I am a member because just about everything is between 3.5 and 4.5 stars (inclusive), too coarse a scale to rely on.
The person I think does a good job at reviews is The Filthy Critic.
The Filthy Critic
I have discovered so many good movies, TV shows, books and CDs at Metacritic that barely a week goes by without me checking it. The scoring method is far better than the Rotten Tomatoes model.
Everyone I have recommended it to uses it now.
Nearly every Netflix rating I see is somewhere in the vicinity of four stars, so they’re pretty useless to me. Rotten Tomatoes is generally pretty accurate. I don’t really pay attention to IMDb’s ratings, aside from checking out their top 100 or whatever it is every once in a while.
I’ll second that. IMDB skews way too much towards fans, RT suffers from the black/white, either/or mentality, and I don’t get Netflix. Metacritic is more nuanced in its approach (using a sliding scale) and more discriminating as well (not every random Tom/Dick blog gets cited).
I’ve found that the reviewers at www.chud.com tend to have very similar tastes to mine (except when it comes to horror) so I’ve begun trusting them for the most part. Metacritic looks promising though.
Netflix uses a 5 star rating, not 4 stars. I should know; I just rated a movie five minutes ago.
movielens is a neat site; it’s a computing project at U of Minnesota, and you have to spend some time rating movies you’ve already seen, but once you’ve done that, it’s very accurate, IME. I’ve only had a handful that I’ve rated significantly differently than the predicted rating.
Another two thumbs up for Metacritic. I’ve been relying on them for a couple of years, for both movies and games. But what is the “sliding scale” of which ArchiveGuy speaks?
Roger Ebert and I – and the wife – tend to have the same tastes. We’re glad he’s returning, albeit slowly.
lol. Yes, thank you for pointing out my typo! You see, the 5 key and the *4 *key are right next to each other on my keyboard. Sometimes I hit the wrong key. Well spotted! 
Rotten Tomatoes gets a percentage simply from a Good/Bad, Yes/No mode of thinking. Which means a 90% may mean 90% of the critics may have thought it was OK/pretty good, but nowhere near as praiseworthy as the percentage suggests.
Metacritic evaluates each critic’s response and level of enthusiasm toward the film in assigning a number value. Sure, it’s a little more subjective, but it also more clearly helps delineate those films that have large-but-tepid support vs. large-and-jubilant support. RT makes no distinction.