I loved Superstar Saga and Partners in Time, but I got Bowser's Inside Story first, so I have to go with that. 
The final SS battle was insanely hard for me, and after I finally beat it, the castle exploded and I was sooo upset. XD I thought it was a really good game, though. I laughed for the majority of the game,
PiT was the easiest game in my opinion; I beat the final bosses on the second try (it would have been the first, but my dad interrupted it and killed me
) The boss music was also really good and intense.
BiS was the game that made me get the other two; I loved the gameplay, the humor, and Fawful. While her tutorials could be annoying, I liked Starlow as a character. She was a sassy female role, and her interactions with Luigi were pretty funny, imo.
I don't have a 3DS, so I've never played DT, but I've heard it's good!

The final SS battle was insanely hard for me, and after I finally beat it, the castle exploded and I was sooo upset. XD I thought it was a really good game, though. I laughed for the majority of the game,
PiT was the easiest game in my opinion; I beat the final bosses on the second try (it would have been the first, but my dad interrupted it and killed me

BiS was the game that made me get the other two; I loved the gameplay, the humor, and Fawful. While her tutorials could be annoying, I liked Starlow as a character. She was a sassy female role, and her interactions with Luigi were pretty funny, imo.
I don't have a 3DS, so I've never played DT, but I've heard it's good!