I grew up with a fair amount of exposure to Paper Mario: The Thousand-Year Door, but I sadly never owned it. I did eventually decide to give the following game in the series a try, however, and Super Paper Mario remains one of my all time favorite games to this day.
Rather than being a traditional Paper Mario game, the addition of the word Super to the title tells you this one plays somewhere in between Super Mario and Paper Mario. The gameplay is a sore spot for fans of the previous entries, but I personally don't mind it. It's not groundbreaking, but it's not bad either. I find the charm of the series to be more heavily based on the stories, characters, music, and dialogue, all things Super Paper Mario excels in.
Yes, unlike most Mario titles, this game has a story. And it's a really good one. It's weird and zany at points, but it has moments of sadness, intensity, and surprise as well. The soundtrack is excellent, and the characters are unique and varied, which is a breath of fresh air in a series that often just reuses the same handful of faces. The dialogue and scenarios are riotously funny, and you never know quite what to expect next. It all adds up for a satisfying package overall, and if you are willing to give the different gameplay style a try, you might find a new favorite here. Highly recommended.