Dell USB-C Monitor (U2520D) connection to an iMac (Retina 5K, 27-inch, 2017) There are many options and cable types that I am mighty confused about the best way to connect the Dell as a second monitor to my iMac. 

I would like the Dell to wake up when the iMac wakes up and I would like to have the best color and resolution that I can have. I assume that using HDMI is not the best way but that some flavor of DisplayPort is better. 

But which one? There's USB3, DisplayPort, Thunderbolt, and then there are several cables. The monitor came with a cable that fits in Thunderbolt but I think is called a USB3 cable. I am not averse to ordering a new cable if that's what is needed.

Can someone help me sort this out?
The only signal that comes out of your iMac is DisplayPort.  So, going from DisplayPort to DisplayPort is the best and most reliable connection as there’s no signal conversion.

USB-C is just a port type.  Thunderbolt is just a multiplexing of several signals:  PCIe, DisplayPort, USB, and power.  The cable that came with your monitor (USB-C) will support DisplayPort so, its perfect to use with your monitor; plug it into one of the two Thunderbolt 3 ports.

Converting to HDMI will undoubtably require an active adapter.  I don’t recommend it as it adds unneeded complexity because the signal must be converted; it’s inherently a point of failure.