Best External USB-C CD/DVD Drive For Use With My iMac

Note that you don't need a USB-C drive, specifically. There are several types of USB connectors that can carry USB 2 or USB 3, and it's not hard to find cables or adapters to go between them.

I bought one of these drives for my Mac.

Other World Computing – 24X OWC Mercury Pro Super-Multi DVD/CD Burner/Reader External Optical Drive with M-DISC Support

It comes with a USB-A to USB 3.0 B cable. Since that cable is detachable, you could plug it into an Apple USB-C to USB adapter … or just replace it with something like this one.

Amazon – Cable Matters USB C to USB B 3.0 Cable 3.3 ft (USB C to USB Type B 3.0, 3.0 USB B to USB C), Black

I have one of these that works great...

https://www.amazon.com/NOLYTH-External-Portable-Desktop-Computer/dp/B0CSPJH9YT?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=ARVX1WX2GE6XO

There is no best in anything. However there is a lot of choices, it all depends on your needs. I would strongly recommend that you visit Mac sales.com as they specialize in Macs and can help you select the right product for your needs. You can call and speak to them too and they can help point you to the device that wlll work best for your needs and for any additional software you may need too.