If you want a great open world to explore I would try out Far Cry 2. It is an RPG more about immersion than customization, and more about having fun during the game than what you achieve at the end. Its not perfect, but I always want to go back and play it for the experience. No other game has made me feel more like I am actually in the game world than Far Cry 2. Beyond the world being great, I enjoyed the combat, and how the missions can be completed in a variety of ways. Some sound simple enough, go kill a enemy leader at a secret jungle base. But how do you get there and what do you take with you? Travel during the day or night? By boat, car, or on foot for a stealthy approach. What weapons will you take? It was always fun to go through missions twice, sometimes I would go with a sniper rifle, LMG, and a mortar gun for a long ranged approach. Other times I would strap a IED to my jeep, drive full speed into a base, jump out 20 yards before impact, watch my jeep hit a few people, then press the button and watch the jeep explode into a billion bits. Beyond the single player, there is a map editor that is amazing. I have enjoyed the many various maps that people have made. One of my favorites has to be someone who converted the Mines of Moria from the Lord of the Rings into a playable far cry 2 multiplayer map. Still the game is not perfect. The story is not all that special, and the NPCs talk kinda quirky, but I paid 20$ for this game, and I think I have maybe two days played total, so I consider that a great value. Bottom line, I would at least try this game out.
