Mass Effect 1 anecdote on a more personal note: I drove the Mako through a reddish, barren, foreign landscape on route to mine some resources. I looked up at the stars by chance and there was another huge planet hovering above the mountainscape in the distance. At that moment some slow music kicked in in the background... fuck, I was floored.
I swear I really felt the weight of the solitude of space (stronger than I ever did in Metroid, oh snap), the barren landscape around me, the breathtaking beauty of that one vista, which wasn't scripted, or had me following a linear, pre-defined path to get there - the fact that such an impressive moment could just emerge naturally out of my own exploration... damn. It's still my favourite moment in both Mass Effect games.
I was really looking forward to more of this in the sequel. I'm really disappointed that they completely removed one of my favourite parts of the first one from the sequel. Hammerhead DLC missions not incorporated into the main game... it's just not the same.