So with this new FPS coming out and it not being anything like the actual X-Com games I want to play one of the originals. They are all 5 bucks on Steam, but I only want to get the best one. So since I know nothing about X-Com, which is the "best" one and which should I get?

Personally I'd go for the original one (the very first one.) It feels archaic now but it's still a great game.

UFO Defense is the game that people think of when you say 'X-com'. That's probably the best place to start.

The first one and Terror From The Deep are the best ones. The ones after those are fun but haven't got the same 'Oomph' as the first two had on me. Ofcourse, I was around when they first got released so I don't know how well they've aged for someone that's completely new to them. 
  
So I say get both and not just one of them.

UFO Defence would be my choice.

From what I understand, the only two worth playing are UFO defense and Terror from the Deep.

I think it has been decided that I will get UFO Defense.

I liked Terror from the Deep, but you better prepare for its insane difficulty.