A non-optimum situation.
All are worthwhile purchases. Battlefront II is a multiplayer-focused shoot 'em up, with somewhat dated visuals but still fun gameplay. Knights of the Old Republic II is the far more conceptually interesting (but also far more broken) sequel to one of the greatest RPGs ever made. Incomplete on release, mods have repaired a lot of the damage and allowed the game to flourish. Dark Forces is Doom with a Star Wars skin, but still awesome. Galactic Battlegrounds is Age of Empires II with a Star Wars skin, but still pretty good.
The two X-Wing games are probably going to be the key draw here. X-Wing vs. TIE Fighter is multiplayer-focused and the multiplayer is a bit flaky on the GoG version of the game at the moment. However, the single-player Balance of Power campaign is included and is very worthwhile. More impressive is X-Wing Alliance, the final game in the series. Released in 1999, the game is the most epic in the series and the largest, featuring a lengthy campaign culminating in the full-scale assault on the second Death Star at the Battle of Endor. It's the most visually impressive of the four games (and also the easiest to update to modern standards through mods), although hardcore fans will argue only the second-best, behind the more morally ambiguous TIE Fighter.
Apparently more Star Wars games are still to come, likely to comprise some or all of the remaining Dark Forces series (Jedi Knight, Jedi Outcast and Jedi Academy), possibly the well-regarded Republic Commando or the more acquired taste of Supremacy (aka Rebellion). We may also get to see the flawed-but-ambitious Force Commander, with Rebel Assault also an outside possibility. Beyond that we are likely to see some more serious scraping of the barrel. It's also possible we will see some of the newer games, like the Empire at War strategy title or the two Force Unleashed titles.
It looks like between these old games, the release of Battlefront III later this year and of course the arrival of Episode VII: The Force Awakens in December, it's a good time to be a Star Wars fan.