Well, this is certainly a controversial, complicated and constantly evolving topic. There are so many bicycle manufacturers, bicycle types and popular brand names sold in the US, that it can be difficult to find a comprehensive summary and to sort it all out.
The top 20 most popular bike brand listing that I've compiled here is not based on the number of bikes sold alone or on the quality or perceived quality of the bicycles. Most of the bike manufacturers listed here also sell different bike types (road, mountain, hybrid, etc.) and the list below is not broken down by bike type or category.
This listing is intended to be a guide as to what bike brands are currently the most popular in the USA, at the time of this writing (since bike brands come and go and the sales of various bike types rise and fall).
The Top 20 Most Popular Bike Brands and the bike types that they sell
Gary Fisher - 29ers, Full Suspension, Genesisters, Hardtails, Kids, Pavement, Road
Ellsworth - High end Mountain Bikes
Klein - High end Mountain and Road Bikes
Jamis - High end Full Suspension, Hardtail, Road, Street, Sport Comfort, Youth
Bianchi - Hors Categorie (HOC), Born for Performance (B4P), Coast to Coast (C2C), Dama Bianca, D2, Special, Classica, Road Steel, MTB Performance, MTB Leisure, Sport, Cross-Terrain
GT - Mountain, BMX, Road, Womens
Rocky Mountain - Mountain, Road, Urban & Path, Ladies Only
Scott - Mountain, Road, Women's Junior's
Some of the popular bike brands listed here can only be purchased at specialty bike shops or independent dealers. Although this marketing technique makes these brands harder to find, in some cases this it improves the perceived quality of the bikes over brands that are distributed to the mass market or that can be easily found at sporting goods stores or even online.
I have included the bike types or bike categories that each manufacturer sells under the brand names listed. The bike type descriptions were, in most cases, taken directly from each bike manufacturers website in order to not only let you know what kind of bikes they make but also how the company perceives the bike types that they make and how they market them to you.