Avoid downloading massive "Mega Packs" from torrent sites or unverified blogs. These often contain corrupted families or files with bad parenting (nested families that break your project template). It is almost always better to download manufacturer-specific content from as you need it.
If you want to skip the reading and get straight to the downloads, here are the three most critical links for MEP engineers: revit mep library link
To get the most out of your Revit MEP library link, follow these best practices: Avoid downloading massive "Mega Packs" from torrent sites