It depends on geographical location.

Chevron is very popular in the west because well, its HQ is in California (right down the street from my relatives :p). On the other hand, it is harder to find here, though not rare. In the Mid-Atlantic and Southeast, ExxonMobil seems to have the biggest presence, and Shell is popular too. 7-11 is fairly big in central VA. Lukoil and Gulf are practically nonexistant. 76 and Arco, popular in the west, are also practically nonexistant here.