I think the best critic working today is John Lahr (The New Yorker), although he can be a bit spoiler-heavy at times.
The worst are Clive Barnes (a hack who hasn't stayed awake throughout a show in years and mostly writes his reviews based on press kits), Linda Winer (her style is a bit too simplistic for my tastes), and the aforementioned Jacques Le Sourd.
Like him or not, John Simon is always interesting. That's what makes him one of the few "must read" theatre columnists out there. The same thing can be said about Riedel.
I also think Brantley is a much better critic than most give him credit for. I think a lot of the hostility towards him is unwarranted. Most people are probably just jealous of his position.
As far as reviews go, they mean nothing to me. A pan isn't going to stop me from seeing a show I want to see, just like a glowing review probably won't get me to a show I have no interest in. I always read reviews, though, because I find them interesting.