This post has a partner post in the Nintendo forum area, so I'll keep the background to a minimum here. What is the most Nintendo game, thing, or decision Sega has ever made, not including when Sega has done contract development for Nintendo (F-Zero GX, Mario & Sonic at the Olympics)?
I'm really torn on this. I could say something like Sonic Shuffle which was clearly a response to Mario Party, and not a great one. There's also a selection of Sonic racing games like Sonic R and Sega Sonic Racing which could be considered responses to the various Mario Kart games, but those don't feel like Sega really making a Nintendo move so much as simply aping a popular Nintendo title, and I think my own inclination is to lean more into when Sega was making either business or design decisions which are more fundamentally Nintendo-like rather than knocking off a Nintendo-inspired trend.
I'm leaning a little into Sega's Mickey Mouse titles on Genesis and Master System. While they do also sort of ape the "bounce on enemies" model of platforming, they do more than just that and the levels feel well-designed enough that it does feel like they were channeling more Nintendo-adjacent design principles. I think I'm actually leaning into either Magic Knights Rayearth on the Saturn or one of the Legend of Thor/Oasis games on either Genesis or Saturn which were developed by Ancient in concert with Sega.