I'd go by dates. Stick with the most recent. Apparently you took Outlook's "advice" at some point and created an archive - this will have stuff not included in the most recent PST file. The files with "backup" in their filename likely contain no new information.

If you want to make sure you don't miss any data, you can open the most recent PST file, then successively import each of the other files. You can set the import to pass over duplicates (watch for the box to check). Also be sure to import to the same folder locations (another box to watch for). At the end of the process you'll have everything in one. Delete all the others.

Future avoidance - Outlook PST files are somewhat prone to corruption as they get huge. My advice is to pick a folder, preferably on another disk, to backup to and regularly (once a month?) backup there using the same file name.