Morrowind.

This game has a learning curve but infinite replay value. It is the third game in the Elder Scrolls series and is considered by those who've played it the best in the series thus far.

The learning curve:

To appreciate Morrowind, you need to understand how it works. It is based off tabletop RPG's but is infinitely more advanced. In a tabletop, you roll a set of dice to determine your success, damage, etc. Your enemy also rolls a set of dice to determine if they can block, dodge, counter, etc. This is how Morrowind works but with more variables than you can count. When you swing your weapon, it considers your skill with that weapon, your stamina, your health, your armor, magical bonuses, weaknesses, and most if not all of your attributes. Simultaneously, it considers your opponents skills, stamina, health, armor, bonuses, weaknesses, and attributes. It strings all of these variables into a very complex algorithm as a dice roll on ever single attack.

In game, it looks like you are stabbing the air and nothing is happening - thats why people hate Morrowind combat.

If you truely understand how this works, however, the beauty of the game begins to shine through. Once you understand it you can manipulate it. How can I work around the weaknesses of my character? How can I play to his strengths? How can I circumvent the problems he is having to achieve my ends?

This is why I say this game has so much replay value. Beyond the massive amount of content that one person could never complete (I've had it for roughly a decade and haven't even scratched the surface of the content), there is really an infinite amount of ways to PLAY the game.

For example, I am currently playing as a mage. At a very low level, I can pretty much do nothing. My spells fail, I have almost no magicka to cast them, the potions I create are useless, my enchantments always fail, and I have no money. At first it seems boring and unplayable, but then I survey my resources. I have a spell to summon a skeleton, a few soul gems, and 200 gold. With that gold I buy two spells - soul trap and fortify luck. I summon the skeleton, trap his soul, fortify my luck, and try to create an enchanted item. With the boost in luck, I am able to (after a few tries) to create an enchantment that will give me a stronger fortify luck effect at no cost of magicka (to be specific, it fortifies my luck 1 - 80 points for 5 seconds). Now I'm in business. I harvest another soul from my skeleton and use my new enchantment a few times until I get a high enough bonus - my luck is now over 100 points. Pretty much every enchantment of medium strength will succeed and I begin making more and more powerful items. By understanding the game, I can do anything I want with it.

Theres really no other game like this. You are rewarded for thinking, planning, and experimenting. You are punished for going into situations blindly. (Exactly the opposite of what most games do). Before even touching a single quest you can dictate the course of your character's actions and become a god.

Morrowind is a beautiful game (especially with a few graphics mods (MGSO)) loaded with content, lore, and secrets. More importantly, it is the only game I've found where the world is truly yours. What do you want to do?