In my mind, Star Wars: Knights of the Old Republic has loads of memorable planets, but I don't know whether it's because they were brilliant, or because this was simply the first time BioWare had done this: take me from one planet and land me on another. Mass Effect wasn't a thing then, remember. It was still a whole Jade Empire development away.
But 18 years later I can still remember Manaan. And by Manaan, I don't mean my nan, but the water planet in KOTOR you dived under the surface of, in search of secrets, only to be chased by huge alien sharks. And there was Kashyyyk, with three 'y's, the forest homeworld of Chewie's race, the wookies. And there was Korriban, the sacred Sith homeworld. A place like the Valley of the Kings in Egypt. A place of tombs and darkness, secrets and treasure.
But there's one planet in the Knights of the Old Republic saga which looms over all others (and I'm stretching out my net a bit here to include the second game too). And that planet is Malachor V.