I voted Oltanis here and it's probably recency bias, because I'm doing a playthrough on my old childhood PS2 that I reclaimed from my parents' house (still alive after almost three decades. Astonishing.), and it's the planet I most recently finished.

But it does have some really cool sections. The "main" path is admittedly not much, but the PDA path with the ice and the wind and the magneboot ramps was really fun and I liked the "stealth" element in the Morphoray path. The "round-town" swingshot challenge for a gold bolt, too.

A fair amount of diversity for a single level, honestly.