I am going to have to say SR388 is the best planet in the Metroid series. I would like to point about that Samus gets infected with the X parasite on the surface of SR388 at the beginning of Metroid 4. So SR388 is not just from Metroid II: Return of Samus.
The planets from Metroid Prime 3 were the least enjoyable because the regions did not interconnect like they on Zebes, Talon IV or SR388.
Games I have Finished