Zebes, hands down. It characterizes the Metroid series and probably holds most of the more interesting locations to date.

Tallon IV may be my second favorite, but it is sorely beaten by Zebes if not because it's not very "Metroid" in presentation (save for the Impact Crater). Places like Phendrana may have been beautiful, but they certainly did not hold that deep-space alien quality Samus is known to treck. If you hide the sheegoths you could actually pass the place off as Earth save for the occasional creepy crawler. Tallon Overworld is similar but it gets a slight pass if not because it loosk to be semi-based off of Super Metroid's above-world Crateria.

Bryyo had ancient ruins, angry natives, hellish fuel gel deposits, exotic flora, bizarre geography and mysterious technology. It was awesome.

The only thing Tallon IV had over it was ghosts. While they're definately cool, I don't think they tip the balance.

Zebes, because it has that pure "not earth" feeling to it.

Tallon IV

There was just so much to explore

Tallon IV :)

Hard to say, it's a tie between Zebes and Tallon 4. There were too many environments in MP3 that looked perfectly made for Samus and not enough gritty, realistic atmospheres.