On the flatter stages, the riders fly by so fast it's a whizzing blur. Instead, opt for the mountain stages, on which the riders will likely be strung out single file in small groups and going (relatively) slower as they labor up the steep gradients.
Whereas on the flats the peloton of 176 riders could shoot by in less than a minute, on the hardest mountain stages there could be over a half hour between the leader and the last rider.
On the biggest mountain stages, which feature several high summits, you'll want to be on the penultimate or final climb, near the summit (but perhaps not at the very top, since it can't get packed with spectators and crowd barriers can limit your movement).
The main mountain regions in the Tour de France are the Alps and Pyrenees. Our favorite climb is Alpe d'Huez — a massive party — but each Tour features different climbs, so be sure to check the Tour website for the general route when it's announced in October and the detailed stage maps published in May.