What are recommended emulators?
Well most importantly they're emulators that are recommended by /emugen/. They usually either fall under 1 of 2 categories: general use, or accuracy. Recommended general use emulators tend to: have high compatibly (though maybe not as high as others), are user-friendly, offer useful features, can be run on various devices, run fast even on lower end hardware, and can generally be used day-to-day by even the not-so-tech-savvy users. Accurate emulators are recommended as they're as close to the original hardware as you can get, and as such are recommended for users who want to recreate the original experience or just want to ensure they'll be able to play most games with the least amount of glitches.
The list of recommended emulators is available on each Console page. Check the console page in Category:Consoles and look at "Recommended" column.
What aren't recommended emulators?
Emulators that aren't the best in those categories. They still may be good emulators, and offer many great features, but it doesn't do people much good recommending every decent emulator under the sun.