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well that depends on which kind of plot you would like, for a more sci-fi mmo, play star wars: the old republic, for a more fantasy sword and magic genre, i highly reccomend world of warcraft, its a lot of fun, with very high replayability. i hear great things about ToR as well.
there are many games out there that are fun. i suggest world of warcraft, star wars the old republic, or league of legends(free).
As others have said, you haven't provided us with any information to what you would want to play. There are a lot of MMO's out there, and it's hard to choose one, but they all fall into different categories, so you can pick from there. As for me, I'm anxiously waiting for Guild Wars 2 and Planetside 2 this year.
Play the one that your friends are playing. MMOS are massive, if you are going to invest the time into them it is best shared with real life friends.
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World of Warcraft is the most polished and one of the largest experiences.
Star Wars: The Old Republic plays a lot like World of Warcraft but has fully-voiced quests and is a more immersive experience.
Those are the two I have played and I can recommend. As was said earlier, look into Guild Wars 2; its really supposed to shake things up.
If you are looking for something different than generic MMOs try:
Fallen Earth - postapocaliptic MMO with good craft system, bikes, cars and simple economics
Wurm - medieval times, specific gameplay and even more specific logic. Most advanced and deepest crafting sys I've ever encountered in games (you will have to create literally everything, even some tools). It's hard game, but all this is rewarding, when you build a fort with friends and roleplay ;]. It's not free, you have skillcap 20% but it's enough to pass some free time and maybe buy it.
Both games had fairly mature fanbase and was generally fun to play.
Guild Wars 2
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I'm pretty new to PC gaming, but I have really been digging Star Wars: The Old Republic. The class (main story) quests have engaging stories and the culture of the universe seems to be well crafted.
I've been looking. At Guild Wars 2 and I've got to be honest, I'm excited. If things are even half as well put together as the screenshots, articles, and gameplay footage would imply, I'm getting it. But when it comes to MMOs or PC gaming in general, I don't know jackshit.
I tried out TOR, it gets boring really fast, either play it safe and do WoW or try out the up-incoming Guild Wars 2.
See, I have never been able to play WOW, I like to actually role play in my RPGs first and dickaround with friends second. WOW is far too much of a grindfest for me to get into lol.
A few yet to come out
If you're looking for a free one, go with Runescape