Competitivecyclist.com and backcountry.com (same company) are generally very good with broad selection and quick shipping. An important tip is to set up an account at activejunky.com and then go to competitivecyclist.com or backcountry.com through the links. You will get quarterly cash back checks from activejunky on most items.
Competitivecyclist used to have coupons that applied to pretty much everything on their site and I used these to buy several bikes in years past. However they have slowly added more and more exclusions to these “site-wide” coupon codes. However they still have some fantastic sales like the recent 12 Days of Christmas sale and the annual Tour de France Sale.
Another great site is Westernbikeworks.com. I discovered them in the past year and got some really nice deals on tires, a Garmin Fenix 5 Plus, and numerous other items. Their shipping is also super quick.
For mountain bike stuff, Worldwidecyclery.com is great. They often have coupon codes but have been a little harder to get as of late. If you go to their site through the mtbpodcast.com site, some money kicks back to Jonathan’s MTB biking podcast and supports it.
Bikebling.com has some great specials but sometimes the shipping is slow. I got a great deal on a Fox fork from them but it took about 10 days to ship after I ordered. I’ve heard they don’t always have inventory and stuff has to come from the manufacturer first. But if you are patient, there can be some really good deals.
JensonUSA.com is worth checking out as is Chainreactioncycles.com
More and more manufacturers are selling direct now too. Enve.com, Trek and Specialized are all examples.
Artscyclery used to be great but they have essentially gone out of business.