Probably the best MMO/RTS on the PC and interwebz.
There is a massive range of characters for you to choose from, all ranging from different classes and type. The best thing about it though is that you can mix 'em up. For example, with some carerful planning you can turn your mage into a melee AD fighter. There are hundreds of combinations for you to explore, and going against each other in the arena not knowing what to expect next is just a complete and utter blast.
You constantly have to talk with your teammates. Where are the enemies? Which lanes are afk? Who are you going for next? What item are you buying? What style are you going for? The difficultly of the game requires consistent commication, and League of Legends does exactly that in a fantastic style.
Building your ranks, buying runes, collecting characters and experimenting is just another way to enjoy yourself here. You can invite your mates into your game and plan a new way to win. The game on average lasts about an hour, so when you see someone with over 100 wins you know that they've earned your respect. The game is very difficult and the other players will be completely without mercy. You can even get reported for dying too much, even with the other character is OP.
The bad? Well, once you start you'll be terrible. It'll take months before you get the hang of it and before you get characters you're comfortable with. But when you do the wait and effort is more than worth it.
Besides, it's free. You can't agrue with that!