okay I've had 2 days to deliberate on this one
My gut instinct is to say the Ember twin since that's the first planet I visited and what cemented the outer wilds experience for me, but I also can't underplay how cool Giant's deep is mostly for the moment you first dip beneath the atmosphere and get hit with a wall of motion and noise from the relative silence of space.
But mostly thinking about this I've come to appreciate that part of the beauty of Outer Wild's exploration really is how each planet provides context for one another. Over the course of the game you can explore the same area multiple times with each time being a new and novel experience because you're primed to be looking for something different or you have a greater understanding of the world at large: very neat.
And now I'm curious just how many square meters of walkable space there actually is in that game

