Obviously I haven't tested all games, but I always recommend the Ace Attorney series for games that run full speed. Pokemon Mystery Dungeon runs close to full speed in most areas and full speed in some areas. Mario and Luigi Partners in Time runs consistently almost full speed, and it manages full speed with a frameskip of about 1 or 2. Oh, but frameskip isn't implemented in melonDS right now; it's something I was playing around with when looking for ways to improve the speed
I can add it in the next release if people want it. And this is all with maximum overclock enabled, of course. Worth noting that overclocking is broken on Switch system version 8.0.0; it's being worked on in libnx, and I'll try to get a new build out soon.