Dark Eldar are great!

As I have them, and love them! However, they are a glass cannon and can be unfriendly to new players. But great overall!
Tau! A little more forgiving, and lots of different ways to play! If you look to get into tourneys, they can be a great start. Also, who doesnt like a bunch of mechs on the battle field.
Nids.... Just got a new codex.... very mean and nasty.... I recently played them with marines and my buddy with necrons. We both got swamped and swarmed. Also, if you like seeing a lot of big mean monsters on the field, that could be a play style too!