Currently, the latest and best selling Apple compatible webcam for iMacs is Logitech's C922X Pro Stream. It's revised circuitry allows higher frame-rates than previous generations of their 9XX series webcams for exceptionally smooth video -- even when there's a lot of motion being captured. It's predecessor, the C920 is still a great external iMac webcam choice, especially at it's now lower street price.
For some, the best iMac compatible webcams to consider will have a standard tripod mounting hole built into the camera's base to allow the use of a floor, desktop or gooseneck tripod for optimal camera angles and positioning. Even if you just perch the external USB HD webcam atop an iMac's display bezel, you'll still have more control over the cam's position than you would the one built in.