TIE Fighter was and remains one of my favorite games ever.  X-Wing is good, too, and I think they both age a bit better than Wing Commander because they'd made the jump to fully-3D models, even if they are somewhat low-poly, and the missions tend to be a bit more complex.  Buying them on GOG gets you the option of installing either the DOS version or the later 90s CD-ROM version which has some visual improvements.  They are both super good, but if you only play one, play TIE Fighter.

Just...just go play TIE Fighter right now.

A stick is somewhat required for the best results; I got a solid Logitech stick that works with Windows 7 for about $30 a while ago.  My poor Sidewinder isn't really compatible...

Also worth noting, but somewhat overshadowed by the X-Wing news, is that GoG now also has Indiana Jones and the Fate of Atlantis.  This isn't the first time it's been available again, but I think the previous time was maybe as an unlockable in Indiana Jones and the Staff of Kings, so...uh...better to just pay $6 or whatever.  One of LucasArts's best adventure games, hands down, and with a bit of replay value uncommon to the genre because there are three different ways to move through the game.

Dix said
...GoG now also has Indiana Jones and the Fate of Atlantis.

Sob. I bought this from a market in town back when I had an Amiga, some time in the early nineties. Lewis and I took it back home eager to get it started. We loved LucasArts adventure games and Indiana Jones so this was a match made in heaven. We managed to get past the theatre/stage intro section and when we had to change disks to get to the next bit it always crashed. It never worked past that point. We took the game back the next time we were in town and I can't remember whether the stall wasn't there or we managed to get a refund but the bottom line is: to this day neither of us have got any further than disk 1.

I've been trying to get Freespace working with all that Freespace 2 Open stuff, but not been having much luck. I played TIE Fighter back in the day, not sure whether it was a demo or some shareware or something but it was fantastic. I've heard that you should play the DOS version over the Windows/CD-ROM version that GOG bundles because it features the original dynamic music engine rather than some linear rip from the films.