I'm thinking about playing through a select bunch of old ff titles, most of which I've played 'till I lost interest or something like that. So which versions are the best? Like would ff IV be most enjoyable with the new remake or just the snes/psx version? And are there any real downsides/upsides to all the different versions of the ffs from 1-6

If you can avoid the PSX versions, do, the load tims are annoyingly slow. Shoot for the GBA version of FF5 and FF6. Get the version of FF4 either on the GBA or the newest one on the DS, the DS version is great. FF3 you'll have to do on th DS, or NES Emulator, and FF1 and FF2 also on GBA. Do it NOW

For old school Final Fantasies, I'd say 6