The first stages of the Tour are deceptive
There's always a few 'rabbits' who jump out into the lead, but the real race only begins in the very, very difficult mountain stages, where the pretenders disappear and drop out, and the real contenders emerge (and Armstrong is without a doubt the best in the world on the hills).
I have read that Jan Ullrich of Germany is one of the few who has a real chance of beating Armstrong, but I wouldn't bet on it...