Mario & Luigi: Superstar Saga is perfection to me. Engaging and unique gameplay both in battles and in the overworld, fun story and writing, top notch music, and great art. It is easily my favorite video game of all time.
Also, shout out to Yoshi's Island, which is probably my favorite 2D platformer ever - the only major issue is how Baby Mario's cry, despite being there for a reason (to teach the player to get Baby Mario back after dropping him), gets old really quickly after you've learned to pick Baby Mario up.
The first Paper Mario is the only Mario game that I can't think of anything I don't like about it. The story is simple but engaging, the characters - even Bowser and Princess Peach - are fun and full of personality, and the gameplay was never annoying to me. TTYD comes very close and there are things about that game that I like more than the first, but ultimately the annoying backtracking in a few of the levels puts it just a bit lower than the first game in my opinion.
Honorable mentions to Odyssey and Sunshine (I know Sunshine has some gameplay problems, but I still love it for its atmosphere and creativity).