As games, I've played 4, 6, and 7. I would heartily recommend them all. As PC ports...
4 is a very well done one. The 3d graphics are rather primitive but they work, and there's some nice cutscenes with voice acting. I would certainly pick it up.
7's PC port, from what I hear, wasn't so hot. I don't know if what they have on Steam is any better. But I guess there are mods out there for it so I dunno. I still have a working PS2 and the original PS1 discs, even a memory card floating around so I've never bothered. There's also the HD remake to consider, though there's always merit in engaging with the game as it originally was before Square adds a whole bunch of shit to it.
6's PC port is...eh. The graphics update really wasn't done as well as it could have been.
Read this for more on that. Not sure how the gameplay and music has been affected, although I know past updates of FF6 really butchered the musical quality. But there may be mods for that.
For my money, I would not hesitate in recommending FF4 for the PC, while for FF6 and FF7, if you want to play them, seek out means to "acquire" a "digital version" of the SNES and PS1 versions, and the means to "play them" on your PC. Also a random smilie for no reason whatsoever honest:
