For me, the best game would be Kotor. The first one. Kotor II was just a fairy "MOD" of the first game, with a few new features. The fisrt game had one of the strongest points in the gameplay that was the Story, the second game failed to create such an interestng and appealing Story.

The Universe to create a Story is big and the possibilities are almost umbumbered, but with that, Obsidian couldn't  match the story created by Bioware.
X-Wing was another great game, that was way ahead of its time, and yet there's no other game like X-Wng these days.