The third game in the Mario & Luigi series. I played the original DS version a long time ago as a kid, so I picked up the 3DS remake for this one to see what was different.
The big selling point of this entry is that Bowser is a playable character, and in fact the main protagonist. At the start of the game he is tricked by Fawful (the understudy of the villain from Superstar Saga) into eating everyone in the Mushroom Kingdom, forcing Mario and Luigi to work with him from within his body to fix things.
This was a really great game, much improved from Partners in Time. As I’ve said previously, I love Bowser, and getting to play as him was a wonderful power trip. He and the brothers had distinct playstyles, including in the overworld, and I loved all the bizarre levels inside Bowser’s body. It still lacks some of the things I liked about Superstar Saga (no magic :( ), but manages to be good in a different way rather than feeling like a downgrade as Partners in Time did.