Honestly, "best" is highly subjective. There are far too many variables for anyone to offer much more than an opinion based upon their own personal experience and needs -- and the specific computer you have is just one more of those variables. That said...
You mentioned a 2013 iMac, which means that you have the older Thunderbolt ports with a Mini-DisplayPort interface (instead of USB-C), alongside some USB3 ports. If "best" to you means "fastest", than you might want to take advantage of those Thunderbolt ports -- but if memory serves, those older Thunderbolt devices tended to be expensive. On the other hand, if "best" to you means either "most cost effective" or "most future proofed" than maybe you'd be better off eschewing those legacy Thunderbolt ports and instead getting a more modern USB-C drive with a USB-3 cable. (Most USB-C drives come with a USB3 cable in the box, along with a straight thru USB-C cable.) Both are perfectly valid options, depending upon your use case.
Another variable is the technology of the drive itself: HDD vs SSD. These days I tend to err on the side of recommending SSD pretty much every time, as they tend to have a much longer life than a spinning-platter-drive, particularly in
anything portable. (HDDs really don't like it when you move them around.)
Personally, I purchased a
PNY Pro Elite 1TB to backup photos from a 2012 iMac, and have been quite satisfied.