Question: what is the best satellite in space

Guy Rixon answered on 2 Mar 2018:

It depends what you want from a satellite. The GPS satellites are probably the most useful to humanity, followed by the earth-observation craft that let us see how the climate’s changing.

If you meant space observatories contributing to astronomy, then, counting only those currently operating, probably the Hubble Space Telescope, since it’s run so long and been used in so many ways. Kepler, the planet-finding craft has changed our view of planetary systems. And Gaia is in the process of rewriting our understanding of our own galaxy.