Your answer struck a chord with me also, I've actually sat down and tried to answer this question so many times this week..
Olsen Olsen hit me hard when I watched Heima for the first time, and it takes me back to when I saw them perform it at Jodrell Bank a few years ago, snuggled up to my husband (who really isn't a big fan at all) as it was dropping cold, watching all the amazing lights
Svefn-g-englar - something about it reminds me of a Bill Viola installation (Five Angels for the Millennium).. maybe it's the sound of water, I have no idea.
In Spring /Summer Góðan Daginn becomes a favourite, because weather/moods. Glósóli after seeing them last summer at Citadel..
Most of Hvarf feels like a boat trip
I find I get a bit misty eyed when travelling home on a train, whilst the sun is setting and Sigur Ros in my ears (as long as it isn't Kveikur)