ROM packs exist in a gray area. Generally, downloading a pack of 500 commercial games you do not own the original cartridges for is piracy.
If you are looking to fill your Android device with thousands of games instantly, here is everything you need to know about finding and using top-tier ROM packs with RetroArch.
The "top" packs listed below are widely discussed in preservation communities. You must own the original hardware/software to use them legally in most jurisdictions.