Breath of the Wild mainly because it broke so many standards for a Zelda game and was so good that people are still talking about the game to this day. I think this is the peak of the series and BotW 2 has huge shoes to fill.
Elite Beat Agents... While Ouendan 2 came out after that game, that's the last we heard of the series, outside of of trophies, stickers and spirits in the SSB series. The reasons why the series wasn't pushed further is because the 3DS came around, which didn't utilized the touch screen as much... and money ^^;
Still, if they would make a bigger game on Switch (because Ouendan and EBA are Nintendo IPs), they could partner up with Sega and adapt the Project Diva system to EBA. Hatsune Miku got a bunch of console games, but instead of tapping the screen, you simply press the face buttons and directional arrows when prompted.
The Switch's extra space could lead to 45 to 60 songs, getting both the Agents and all Ouendan cheer squad members (Ouendan 2 had 2 squads, 1 being the new rivals to the old ones), The button prompts wouldn't obstruct gameplay as much as those found in PaRappa the Rapper, DDR and the Project Diva games, and DLC would be added by the truckload.
EVEN BETTER, the Ring Fit ring could even be used as an alternate controller, just like Ring Fit Adventure added a new rhythm game mode this year... which completely flew under the radar due to the pandemic... and the surprising Ring Fit shortage XD