I'm torn here. In terms of potential, post-reformation Spherus Magna sounded incredibly promising with all these disparate factions suddenly sharing a planet again and being forced to explore and resettle it in the aftermath of the GSR fight. Plus, it could have been a good way to return the increasingly-sci-fi series back to the original tribal feel. But the serials never really got to do much with it, and what they did do I wasn't always a fan of anyway, so whoops...
In terms of concept, both Mahri Nui and Karda Nui get props from me. One's the place where the Toa originally came from, with the secrets of their past... But in execution it didn't amount to much more than some rope bridges between rock towers over a swamp, without much in the way of specific landmarks or personality. The other is an underwater city over a deep trench, which should absolutely be getting my attention, but it was similarly underdeveloped.
In terms of execution, Metru-Nui's pretty great, taking what could have been really boring after the genre-mashing of the early years (robots in a robot city woo) and fleshing it out with an Orwellian society and a dark, noir-y vibe, serving as a pretty solid backdrop to the Toa Metru's struggles. Revisiting it in '05, ruined and covered in webs, created an entirely new atmosphere from an established location, and really rubbed it into the Toa's faces what they'd lost.
But easily the most iconic and my favorite is of course Mata Nui. What can I say? It just worked so well on so many levels, with the most developed culture and cast of supporting characters, if you aren't overly picky about what counts as 'canon'.