Oh here's an obvious one I can't believe I didn't post before:
First I'll start off by saying that after a recent replay of Sticker Star I'll agree that one doesn't seem to hold up that much anymore. It's got some good levels and such but eh fine I'll agree it's the "worst" Paper Mario. Color Splash however? I'll defend the hell out of that game. Yes it kept most of Sticker Star's mechanics but eh I dunno I felt CS handled them better. Also I in general felt CS was a much better game and would go so far as to rank it alongside Super Paper Mario (a game that may not be an RPG like the older titles but it's still great). Huey was a very likable partner, it fixed issues I had with SS (for example I did think it was stupid Bowser never spoke in SS so having him and other villains be more talkative helped make the story more interesting), and I can only think of maybe one or two levels in CS I didn't like, while in SS there was probably an equal mix of "like vs didn't like."
And so far what I have played of Origami King (I'll confess I haven't beaten it just yet as of typing this comment) I'd go so far as to say it's the best of the "Tanabe era" Paper Mario games as it further fixes SS and CS's flaws (basic jumps and hammer aren't one time use items, and even if there's no XP the game makes up for it by giving coins a greater purpose and the accessories are basically just the return of badges), and delivers what is probably the best story in the series since the original trilogy of games.