While the upcoming Final Fantasy XV release is still slated as console-only, SquareEnix has been bringing a substantial portion of their back catalog to Windows. The company seems to be slowly marching the Super Nintendo era toward Steam, just recently announcing that Final Fantasy V will join III and IV on September 24th. This leaves just Final Fantasy VI missing from that era, at least from the main series, which suggests that it will join the party (pun intended) in a little while.
A few other titles are still in radar silence. The entire NES era, Final Fantasy I and II, is no-where to be found… unless you count the former's re-release on Windows phone (although, even if you do, a case for “no-where to be found” could still be made). From there, everything has made it to the PC until you reach the aforementioned Final Fantasy VI.
From the PlayStation generation, both VII and VIII launched on the PC back in the late 90s, and both have been re-released on Steam, so those are fine. The only missing title is Final Fantasy IX, which is currently an original PlayStation exclusive. It has not been remade for any other system, period. This is a bit concerning, because it means that a team cannot be set aside to bulk-port a chunk of titles. Every port from that generation stemmed from their PC versions, so this would (at least I expect) need to be a special case. It never had one. Would they think the effort's worth it?
Next is the PlayStation 2 generation. This is a PC dead zone, apart from Final Fantasy XI, the MMO, which launched on Windows alongside Sony's console. We need ports of Final Fantasy X, X-2, and XII for the platform to be complete. Interestingly, the PS4 has just received an HD remaster of X and X-2, but XII is stuck on the PS2 (at least for now).
This brings us to the PS3 generation. The only thing we're waiting for is Lightning's Return, which is the third installment of the Final Fantasy XIII trilogy. It has been announced and, in fact, should have already launched several months ago. SquareEnix has confirmed a delay, re-affirmed that the PC will get it, but a firm date has not been set. Still, I'll count it as “PC”. Final Fantasy XIV was an MMO that launched, a few times, on Windows.
Lastly, Final Fantasy XV and Final Fantasy VII Remake may or may not come to the PC. Who knows?
So, ignoring the offshoots, we are currently missing: Final Fantasy I, Final Fantasy II, Final Fantasy VI, Final Fantasy IX, Final Fantasy X, Final Fantasy X-2, and Final Fantasy XII (plus the future titles). It is funny how SquareEnix seems to be grouping the ports by generation. While it looks fairly random from the Steam search page, the gaps make sense when you consider the work required to port a game. Ressurrecting Final Fantasy IX is a completely different process than VI.
Final Fantasy V will come to Steam on September 24th. Some may argue with the price, but you can wait for it to come on sale if that is an issue. You've waited long enough already.