Sonic Mania runs flawlessly on New 3DS besides special stages having a lower framerate and longer load times. I think they made it available for old 3DS somewhat recently but Idk how well it works

ok thx

i'd like to say undertale DS but sadly it runs like ass.

Sonic Robo Blast 2 is another homebrew port that runs well. Half-Life... is OK, if you ignore the loading screens and performance. At least someone picked up on the project and works on a fork that even introduces stereoscopy (3D), so that's something nice to look forward to.

Also, Mario 64 port. It runs like a dream and works great with C-stick on n3DS.