Metro is about as on-rails as you can get? It's all "go down this one tunnel and shoot everything" or "follow this actually quite narrow and linear path across the rubble and shoot everything". Some of the missions are literally on rails (you're in a subway, after all)
Far Cry is much more open than Metro, and often has some quite fun things to do if you stray off the mission path and talk to the right person. Depends how much you like to hunt and fish amidst blowing up the bad guys!