The best external drive will be the same as the best internal drive ... putting it in a box changes nothing.
I don't recommend buying an "external" drive tho ... I think you will be better served using a HD docking station such as these:
https://www.newegg.com/Product/Prod...&Description=blacx&ignorear=0&N=-1&isNodeId=1
Then you can just buy internal drives and pop them in and out ... get one for the additional space and one for offsite storage... only one "box" required. You can also pop in ant drive for troubleshooting.... much eaiser to fix a boot drive by popping it out of the puter and disinfecting or fixing it by popping it into the dock on another puter.
You can make images of drives and if an installed drive goes south, just open ya case and put the imaged drive in.
As for which internal drive to pick, we haven't bought / installed a HD in 8 years. All or boxes have SSDs and we use SSHDs for the bulk storage needs. They are of course much faster than HDs...
Boot Time SSD = 15.6 seconds
Boot Time SSHDs = 16.5 seconds
Boot Time HD = 21.3 seconds.
Gaming Score Seagate SSHD = 9.73 MBs
Gaming Score WD Black = 6.23 MB/s
Of course budget has to come into play... we talking 2, 3 or 5 year warrantees ? ... 5.400 rpm or 7,200 ? ... High performace or slowpokes. ? I'll assume 5 year High Performance
Failure Rate Seagate SSHD = 0.43 %
Failure Rate WD Black = 0.45 %
Cost 2 TB Seagate SSHD = $99 MBs
Cost 2 TB WD Black =$120 till tomorrow ($159 after)
Also, what kind of dock / I like the doubles that handle two drives (2.5 and 3.5) so it's easy to dupe a drive.... also what interface USB 2, USB 3 or eSATA... or WiFi ... Select accessories storage / docking station from the drop downs
https://www.thermaltake.com/accessory.aspx
https://www.thermaltake.com/products-model.aspx?id=C_00002325