Probably Freelancer it's going to sound weird because supposedly the Elite/X games are better, but I've honestly never enjoyed them as much as I liked Freelancer.
Basically flying about in a ship, doing missions and upgrading weapons etc until you can buy a better ship, online was fun as well.
NMS was what I hoped would be the freelancer part 2 but it's just an exploring game. While fun on its own, completely lacks any real danger and that just kills it for me.