“Strings of Light” is gorgeous, but I think my favourite (so far) has to be “The Last Voyage”. It’s a neat exploration/strategy/resource management game, but the stories it tells along the way (with the help of relativistic time dilation) are what make it so great.
“Catapult, Cat Space Program” is also great—similar gameplay to “Strings of Light”, but it takes a silly rather than introspective path. And it’s awesome.