It's quickly becoming Thief. The stealth mechanics are a little less clear-cut than the original series, but I'm very impressed by what I've played so far. Stealth mechanics in games tend to be very simplistic, and the Thief series is arguably the greatest stealth series ever made, and the first chapter of the new one feels like the old ones, right down to Garrett's voice and the subtle sense of self-satisfaction as he burgles his way through the city.
