Play many MMO, this is by far the most solo friendly of all. Unlike other MMO, you don't even need to team up to fight stronger enemies or boss character. Any players who are around you, will automatically become temporary party members until you kill the boss or any enemies around you. You will still need party members for dungeons, but you don't even need them to get good loots. WvW is the best part, you don't even need to know anyone to participate. Anyone who is in the same server as you will automatically become your teammates until you left the game.
If you are love to play games solo, looks no further to GW2.