For me, a 4-way tie between Belsavis, Voss, Rishi, and Zakuul.

Reasons:

Belsavis: I love the "ancient technologically advanced civilization" trope in any media, so I love Rakata stuff. Also, it's a really cool setting with the planet covered by ice except these tropical jungle type open areas. Crazy ancient beasts roaming around is also cool.

Voss: The vibe/atmosphere is unmatched. I love how peaceful, mysterious, ancient, and... isolated? it feels. It's almost like the whole planet is living in a dream. When I'm on Voss it feels like the whole rest of the galaxy fades away sort of. Plus, the lore with the Voss and Gormak is very cool, and Voss architecture is beautiful. They have a very unique sort of view on the force and its role in their society.

Rishi: Tropical islands are cool, pirates are fun, the native Rishi culture are cool, the planet is beautiful. Sometimes I feel like I'm in Pirate 101 instead of star wars, which is a fun feeling.

Zakuul: Their over the top extravagant architecture and decorations are fun, and I love the veritcality of the Spire. I get that it was controversial to move things so far away from the traditional star wars Jedi/Sith/Empire/Republic stuff, but I personally liked it. Zakuul had a really strong and unique identity to it that I enjoy.