I'm partial to Scuba.com. I coincidentally have the good fortune to live 20 minutes from the headquarters. They have a show room you can walk into and then "order" gear/equipment from computers at a table. Instead of shipping to your house they bring to you in store. They also have a great return policy. I had to order a wetsuit online from the email to get a special email only deal (wasn't available in store at the sale price) but the wetsuit didn't fit and had to take my daughter back for a return. They gave us full credit and spent the next 30 minutes bringing out different suits for her to try. They've also taken back a pair of fins after several months. I ordered them for her but we didn't dive them and then she wanted a different color. They took them back no problems.
I've also used Scubatoys in the past but haven't needed to return anything to them.
Scuba.com also has a complete service department, and is an authorized repair/warranty station for most major brands, including Atomic. Tanks are filled on site, and only ten minutes from my office.