Mine would have to be a combination of Dantooine and Korriban, especially as they are polar opposites.
Dantooine is so peaceful and relaxing and is also the place where you first get your Lightsaber. The Ancient Ruins are also really interesting and meeting people like Juhani and having your first epic Lightsaber battle is pretty cool, also the Crystal Cave is always fun to go and explore.
Korriban is also a great place to go, the whole concept behind your visit to this planet is what grips me; infiltrating the Sith academy and ultimately all of the tasks are fun to carry out. Even helping Yuthura and Uthar Wynn is enticing enough to keep you interested. Also who doesn't enjoy going through all of the Sith Lords tombs and finding really powerful items.
My least favourite would have to be Manaan, I liked the planet but the whole underwater sequence drove me insane. More specifically the ultra slow walking you can do when in the water, I mean I know why it was done; for more realism, but it got really boring really quick.