Mine is probaly zebes because that is where I started the metroid games.

I liked the sky town in Corruption. Zebes is sick though.

I also like bryyo.

Bryo was great, and so was Elysia (MP3). I think Prime 3 had some of the best planets in the series.

i really liked the sky town but i also like the snow one in metroid prime(i dont remember the name)

 You mean talion IV and the snow part is phenandra drifts.

Yes, but the "snow one" was not a planet. It was a part of the planet Tallon IV.

Talon IV was just an incredible planet. As much as I liked Prime 3, I think I prefer Prime for it's art design. It was just such a lush and natural world.

^ yep all natural except the phazon mutated parts and phazon mutated animals. Other than that it was really cool and the reason why it was more in depth is because you stay on that planet the whole game.

I also like phaze that planet is the best living planet in any scifi franchise.

The new planets from the Prime series are much more planet-like and have unique, interesting characteristics, etc.  But Zebes will always hold a special place in my heart because I was like 9 years old when Metroid was released and loved it.