Dream Team, easily. I'm a huge fan of the Mario and Luigi series and the Mario RPGs moreover, and after the disaster that happened with Sticker Star on the same system, Dream Team - despite its flaws - was a highly enjoyable M&L game and for over a decade until Brothership was announced it was the last Mario RPG, not counting remakes, that was on the same level as its predecessors, and had a wealth of original characters, unique setting, and story that weren't blatantly restricted like the modern Paper Mario games. It also has a fantastic soundtrack and incredible visuals for a 3DS game.