I'm 31 and I've played about 30 MMOs, and so many thousands of games that I can only really say I've been a hardcore gamer with Console, PC, Handheld, Pen & Paper style like Dungeons and Dragons, made my own games, and played games made by friends that no one else will ever see.
That being said, Mass Effect is a definite jewel in storytelling, as even erasing Shepherd from the story and all gameplay, just the socio-political interactions between the fantasy races and the history within the game's universe can be talked about by fans for years, which has already happened. Even without playing it or having interest in games, the story can and has brought other people who simply overheard the story to find interest, even if just basing certain races psychology on our own and seeing how they would react given certain situations. If you can be thorougly entertained by a game's story without even acknowledging the game aspect of said game, I say that's a huge win for your essay's purposes.
Another game I find very impressively written is The Legend of Mana. Unlike any other game I've played, there are three different MAIN storylines totally unique to each other, and beating any of those quest lines gets you to beat the game. However, each of those three quest lines are emotionally involving, and unlike other games, the side quests don't feel like side quests (except the quests for Bud and Lil' Cactus). The first time I beat the Jumi quest line, I cried. I find that a well-told story.
If you tie together Dragon Warrior (Dragon Quest) 1, 2, and 3, then that's a great story too. The other games in Dragon Quest don't tie in with the rest though. In the first game, you find the hero "Edrick"'s equipment in order to save the world. In the third one, you can have incredible armor and weapons forged for you out of unique and powerful materials. At the end of the third game, the king gives you the title of "Edrick", which explains "who the F is Edrick?" and why that equipment is so awesome in the first game. Only by playing the three first DQ games could you get the emotional satisfaction and intellectual enjoyment out of it, otherwise it wouldn't mean anything.
Metal Gear by itself not so much, but just like Dragon Quest, play at least the first few games. The story gets richer and more convoluded than just in the first game, and it is very understandable even though its an extremely complex story; partially or entirely because it has to do with the behind-the-scenes workings of the government.
One of the other 'best' stories ever told, I say is Ultima Online. The starting movie is great in its openness. You create your character, start, and who knows what after that. No big overarching story or objective, you set your own agenda and goals, and each server has different issues and history- especially since the game came out as the FIRST MMO in 1997. Since then, the EM (event moderators) and GMs (game moderators) and even players have created villages, storylines, and the world itself is permanently different based on player actions within that server. No server has the same storyline now, they're all different as well as the landscape in many places. To my knowledge, Ultima Online is still the only MMO to allow towns to be permanently destroyed and rebuilt elsewhere if the players didn't band together in the town's defense.
