I am not going to get into the discussion of whether the USB ports should be on the front, side, or back of the iMac. I think it is a preference thing. If you want USB ports on the front you have the option of adding a single port with the iMac Jimi USB Extension or now you can add 4- ports with the Satechi Aluminum Clamp Hub.
I like the design of the Satechi Aluminum Clamp Hub with 4-Ports (ST-UHA3CL, $30) because it doesn’t require a hub to be laying around on the desktop and is made of brushed aluminum to match the iMac. The USB ports are 3.0 and don’t require any software installation. Clamp it on and plug it in. Done.