The DS game was peak. The style was completely new for 2D Mario, it felt connected to SM64DS via the minigames, sound design and reuse of most models, and a good chunk of its bosses were unique enough takes on enemies to feel straight out of Paper Mario.
Dry Bowser was a novel idea that was genuinely unexpected at the time (he'd later get repeated enough to lose that impact but here it was cool), playing as Luigi being accessed via a classic cheat code style method rather than just a basic menu, and it even took a 3D Mario boss and adapted it into a 2D boss fight in Petey Piranha, in addition to the original bosses. I should replay it sometime, that'll be a nice nostalgia trip.
The Koopalings returning in NSMBWii was novel at first. Then they did it again, and again, making all the non-DS games feel like just repeats of the same thing. I know World had also reused the Koopalings from SMB3, but idk I don't think that should have been a precedent for repeating them to an even more derivative degree in the NSMB series.
Objectively, Wii and U are the games with the most replay value thanks to their multiplayer co-op, but I'm a shmuck that never had friends to play with and my fondest memories remain of the DS one. The others blur together in my memory so much that you could show me certain levels and I literally wouldn't be able to definitively say whether they were from Wii or U.
2 is just Wii and U except with a fucked economy instead of multiplayer. In some ways it's similar to DS, but not in any of the ways that made DS my favorite.