It depends what games you want to run. For most 2d and 2.5d DS games Desmume is your best option (https://desmume.org/).
If you wanted a emulator for older games that can run at 1000% of normal speed then Visual Game Body Advanced is a good choice (https://romsmania.cc/emulators/gameboy-advance/visualboyadvance-68).
For newer DS and 3DS games I would recommend Citra (https://citra-emu.org/).
For switch games I have used Yuzu for games like Pokemon sheild though it is not the most stable emulator but is constantly updated on the GitHub. (https://yuzu-emu.org).
In terms of finding hacked roms to test out these are good (https://www.romhacking.net/hacks/) and (https://www.gbahacks.com/p/pokemon-rom-hack-list.html).
Hope this helps!
