Trek. Everybody has a Trek. Everybody's neighbor has a Trek. Every time I see a road bike, it's a Trek. Every time I see a young kid bombing around on a MTB, it's a Trek. Grandparents out enjoying a leisurely ride on comfort bikes, are on Treks! Sheesh... They are all over the place!!!
The second must ubiquitous brand after Trek is Specialized. Giant, Fuji, and Cannondale are around but not as common.