For desktop drives (which have a power adapter and need to be plugged into the wall): G-Drive are very good, and also Seagate. These are platter/spinner drives, and come in much higher capacity than portable/mobile drives, which are bus-driven (they just depend upon the computer's power).
I use Samsung's external SSDs, the T5, and also HGST/G-Drive mobile external SSDs for many of my current backing up and supplemental drives needs, and then reserve a few older external spinner drives for archival drive purposes. I have two 8 TB Seagate desktop external drives on the workstation now that handle archival storage and also regular backups. One really needs more than just one backup drive; the general "rule" is three: one on-site for current backup, a duplicate of that on-site and then a third somewhere off-site. My safe deposit box serves as the repository for safekeeping of my data (because of their small physical size, external SSDs work very nicely and fit into the box without any problem).
I don't use Time Machine so that is not an issue for me, but if I did, I would choose a larger-capacity desktop external drive, especially since your iMac is not a mobile machine, it's not going anywhere. G-Drives come already formatted for the Mac. Other drives come formatted for use with both Macs and PCs, but it is simply a matter of at the first use, opening up Disk Utility and formatting the drive there to your preferences (APFS is what is optimized for the more recent versions of MacOS, and it is also optimized for SSDs as opposed to spinner drives.).
