Cheers, Guido, like I said I'm just getting into them, any advice as to favourite albums? There's about six or so..?
The album that did the trick for me was "()", still my favorite one. Popplagið (or "Untitled 8") is part of that album - a killer track and the regular closer of a Sigur Rós concert (you get an idea why if you watch the following clip):
http://youtu.be/gZVw6rHw5jQThe most accessible album (in my opinion) is Takk, which also features the well known Hoppípolla (you mentioned the instrumental version in one of your previous posts). Some really great tunes on that album, too.
I also recommend to give their DVD "Heima" a try. Great (stripped down) versions of some of their best songs played live in sometimes strange but beautiful locations all over their home country Iceland.
Good call on M83. Only got two of their albums, 'Before The Dawn Heals Us' and 'Saturday = Youth', really like them both. Can't believe I didn't bother to see them at a small club a few years back when I had the chance. 
I saw them live in Cologne in 2012. Superb gig. Back then I uploaded some clips I recorded to YouTube:
http://youtu.be/TP3rydoIEmMhttp://youtu.be/GKQiUaOgCxAhttp://youtu.be/g8-nD1dBcQQTry also their (or better: his) latest album "Hurry Up, We’re Dreaming". Great stuff.