This may sound like a joke but I'm going to recommend you try Powerfix Profi before forking out for anything better; this is on the basis of a large set of files and rasps I got in Lidl approximately 10 years ago. If they're still the same quality they represent some of the best value out there, sort of like the file equivalent to the Aldi chisels.
I've accrued a very large collection of quality vintage files, but in terms of files bought new these are the ones that impressed me the most, not the Blu-Dan, Swix, or NOS made-in-America Nicholson that I've purchased since (each of which individually should have cost more than the entire Powerfix set).
The double-cut hand file from that set has been my primary metalworking file for all large jobs and it only started to show its age after years of irregular use, mostly light but occasionally heavy. It was noticeably blunted after a big job shaping something in tool steel that wasn't fully annealed so I'd been asking quite a bit from it. I vinegar sharpened it at that point to give it a second lease on life and while it wasn't like new again it was pretty good, and I've had another two years from it since. It just did some major work on a beat up London-pattern turnscrew and it's time for another sharpen, see if I can't get one last stretch out of it before I convert it into a scraper.
However, if you don't have to buy new, don't need a set of files all at once, and don't mind them not matching, buying vintage is the way I'd go to build up a good set. You can get files for next to nothing that if still made would cost an arm and a leg. Even if they need cleaning up I think you can't go far wrong, not at a quid or less per file.