“best” depends on the particulars of your situation. Cloud backup is one of the easiest but over time can be expensive. In the long run buying a second same-sized drive is cheaper than online backup, but it requires more money up front, and having the original and backup in the same physical location doesn’t protect against local disasters like a waterpipe bursting flood. There are specialized tape drives for backups, which are cheap per mb and so you can make lots of separate backups which makes your data safer, but they’re very slow to read and write. And there’s other option too, like optical disks, raid arrays, etc.

Best i can really say is to do some online research to figure out what’s right for your particular case.