I don't want to lose all of my great downloads or the families and lots I worked so hard on ....
Which Folders Should I Backup to Keep Them Safe?
GameData holds a lot of stuff you have downloaded. There are seven sub-folders custom content gets put into.
Walls, Floors, Roofs are all in there and if you use an auto installer (like file maid) objects would be found in the UserObjects folder. Skins folder has your skins you have downloaded. If you are using hacked globals they might be found in the gamedata/globals or gamedata/objects folders. (Personally, since I have the harddrive space, I just copy the entire GameData folder out.)
Since different hacks get installed in different ways you may have hacked globals, game patches, default replacement, etc in the ExpansionPack# Folders instead of GameData/Objects (ExpansionPackShared counts as one of those)
The Downloads folder is where people usually put objects they have downloaded.
Buyable skins can also be found in the SkinsBuy folder. Not sure where that would be in the Complete Collection. Check the ExpansionPack# folders for any SkinsBuy folders starting from ExpansionPack3 and up.
Each Neighborhood is one of your UserData folder. Houses (all lots, including none residential), your actual Sims (characters), their screenshots (photo album) and the FAM files are all in there along with photo albums (screenshots). The numbers in the foldernames correspond to the neighborhood number (UserData5 would be Neighborhood 5, etc). and the first, un-numbered folder is Neighborhood 1.
There are FAM files are stored in the Import folder which is only UserData (Neighborhood1) so to save your family information be sure to save UserData along with the individual Neighborhoods you opt to save, even if you are not using Neighborhood1.
The only other thing I can think of that people customize is Music though you might have some custom stuff in Sounds as well (PiSimSounds objects install there, I believe, and the TV stations are there)
Related Article: What's Where in your Game's Folder?