Mario vs Donkey Kong
DK steals a bunch of mini-mario toys and you have to recover them all. In the first part, you collect a key and go to the second part of the level inwhich the mini-mario is waiting to be retrieved. On the second part, the mini-mario itself holds the key to the next level, which can be a pain in the ass since it follows you around and is a fragile as Mario (one hit and you're done). There are also 3 presents you can gather in each level to beef up or score to finally earn a star for that level. The stars unlock bonus levels which are waaaaay more difficult. Add the mini-marios-in-a-box bonus games and boss battles and you have one hell of a game. The nod to the classic Donkey Kong Arcade is pretty obvious.
Mario And Luigi: Superstar Saga
First off, let me lay out something: the brothers are very gay in this game. Yes, the dancing they do while waiting for an order is ridiculous and so is their victory pose. With that out of the way, this is a great game. The references to Super Mario RPG are everywhere(you even find Geno). You learn abilities such as the jump, hammer, gloves, that you can use in battles and make awesome combinations or in the world map to reach previously unaccessible spots(the electric glove actions is pretty funny).
Kuru Kuru Kururin
You are a spinning stick struggling to leave a maze without hitting anything. Enough said, go play it.