For Kotor One, I guess it would have to be Taris. The planet, in terms of modules, is so large and full of a vast array of NPCs to make it feel alive. All the characters on Taris were unique (even if you literally saw clones of them all over the place) due to their beautifully written story and VO acting. Every level you progress to is different just like how every character describes it as, from the rich Upper City, to the Poor and Crime Ridden Lower City and the frightening Under city. During my first play through, I felt almost exactly like the NPCs in the game feel in their levels: Upper City is a safe area to hang out in the shops and Duel Ring, the Lower City made you fear for your life as seemingly every corner had thugs ready to murder you and the Under city made me literally not want to leave the Village because I thought the Rakghouls were A: OP asf and B: I believed would get the Rakghoul disease.
For Kotor Two: I'd have to put my money on Dantooine, whilst I wasn't as Naive back when I first played Kotor 1, I really felt the same atmosphere I felt with Taris but with Dantooine. The VO and writing cements the concept that this is a run-down, seen better days, poor, weak planet which has little to no law keeping the innocent Settlers and farmers safe from greedy Scavengers and blood thirsty Mercenaries, the plot of the planet is short yet still fun and interesting.
Other planets like Onderon literally forces you to play both Dxun AND Onderon without any option of leaving before you finish your main quest WITHOUT Atton and once you do finish the quest you need to go back AGAIN to fight in the Battle of Onderon. Nar Shaddaa is slightly better but makes up for it with other minor flaws to ruin it, compared to other renditions of Nar Shaddaa in SW games this Nar Shaddaa is boring asf. When I do try to finish all of the bonus quest of Nar Shaddaa I ALWAYS get Attons bloody "We got this message on the Comlink. Looks like trouble" notification which blocks me from finishing any quests until you finish the planets main plot, here I usually just use the "warp" command to get to wherever I need to go just before I get that notification.
Then you have Korriban, which is short and somewhat bland if you don't have M4-78 (Which just makes Korriban a necessary inconvenience just to get to M4-78). Fighting those Hississ' with that bland fighting music and those horrible sound sets, that noise will haunt my dreams.