To start with you'll have to deal with the onboard thermal sensor within the HD as once you take the drive out your system will ramp up its fan speed as its missing the input into SMC. You'll need to get this in-line thermal sensor to over come this with any drive you put in: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later. The second issue you'll have if you put in a SATA SSD is the original HDD is 3.5" and the SSD is 2.5" so you'll need an adapter frame to hold the smaller drive.
OK, on to the differences of the Samsung drives you've listed, I assume you are focusing on the 2.5" drives here. The EVO 850 is the older drive in your collection it is a good drive I've put in at least a 100 or so in many Mac systems. My system here is running on one now (2012 15" MacBook Pro Unibody). The EVO 860 was a slightly cheaper drive as the EVO 850 technology was more expensive and Samsung had to fight off some competition and is the newest version of the 2.5" SSD
EVO 960 and the EVO 970 are both M.2 blade SSD's which will not work in your system.
Now it does turn out your system supports a PCIe/NVMe blade SSD which is much faster than the SATA interface your current HDD is using. So you could install the custom blade SSD instead of the SATA drive but its a lot more work. Here's a good reference: The Ultimate Guide to Apple’s Proprietary SSDs while not complete it's the best thats out there.
Now what would I do?? If you can swing it I would put in the blade SSD 256 GB and also replace the HD with a hybrid drive like Seagate FireCuda SSHD. Then setup the SSD as your boot and app drive and then use the SSHD as your data drive.
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