They're all excellent games!
If you want more of a relaxing fun casual rpg game that lasts you for quite a bit then choose Mario and Luigi
If you want more action and fighting and probably a better plot and more characters choose Kingdom Hearts.
If you want puzzle and logical thinking games then it's Professor Layton.
For me I would choose Mario and Luigi because it's not hardcore and just fun to play whenever you get the time. Kingdom Hearts though is a great game and the plot is really well made!
Note: Mario and Luigi Bowser's Inside Story does not have wifi or DS download play features!
