I own Star Wars Jedi Knight II on Steam, but it was such a waste of money. Not optimized well at all. How is the gog release? Also, is the first Dark Forces any good? I remember it vaguely from when it first came out and playing it at a friends. That was like when I was 9 or 10. Wow.
I have both Dark Forces and Dark Forces II: Jedi Knight (which also includes the expansion Mysteries of the Sith) here on GOG. I did buy the first Dark Forces on Steam, a while back before it came outo n GOG. And yes, you are right at how bad it optimized for modern systems.
On GOG, both games run PERFECTLY! They are optimized really well for modern systems. I am still in the middle of my first ever playthrough of Dark Forces, where as I have already played Dark Forces II and beat it a couple times several years ago and I haven't finished Mysteries of the Sith yet... Then again, I had Dark Forces II and Mysteries of the Sith on disc and I still do. But overall, the first Dark Forces game is basically a Star Wars version of the very first DOOM game. And Dark Forces II/Mysteries of the Sith is more like its better known sequels (Jedi Knight: Jedi Outcast and Jedi Knight: Jedi Academy).
So, I strongly reccommend playing through the games in order as you actually get to witness the evolution for the character Kyle Katarn. But, since you asked which Dark Forces game is the best, then I'd have to say go with Dark Forces II. Nothing beats an engaging narrative with great gameplay, and you can never go wrong about the live -action cutscenes :)