Most 'traditional' (= vintage) folders have reasonably good viewfinders, especially the German ones - as one poster has already written, I also cannot think of a folder with frame lines. Others may differ, if you know of any please post, to satisfy our curiosity.
If your reason for wanting a superior viewfinder has to do with your vision, you could consider one of the older (circa 1950) Zeiss folders - mine is a Nettar 6x6 from about 1950 - with the flip-up/down viewfinders formerly known as Albada finders. Mine has one, and I find it vastly better than the peer-thru finders in the newer and often more expensive models.
(Added later) I see bbutterfield (#4) has already posted about the Albada. Apology for the obvious repetition, I do this a lot of late in my dotage. At any rate my eyesight is far from perfect, and I find the Albada is more useful to me than the built-in finders on my other folder (a Perkeo I).
Or consider buying an external clip-on viewfinder. I believe Voigtlander once made some super good quality ones. In fact I think I had one of their 6x6 viewfinders once, but I no longer have it, and that was so long ago I cannot really recall if it was a Voigtlander or not. Again, others here may be able to improve my too-overloaded (alas, with mostly mundane ideas) memory on this, many thanks in advance.
(Added later) It's called the Voigtlander Kontur. See #13.