What is the most standard way to structure a Unity 2D project? I’ve been looking for the most efficient and organized folder structure but no one has a solid answer on this. What do you personally use?
Nobody has a solid answer because everybody likes to do it differently 
For a 2D project I’ll at least have something like this:
Audio/FX
Audio/GUI
Audio/Music
Levels/<number.name or just name>
Levels/Menu/ (if there’s a main menu scene)
Scripts/ (core scripts go in here, like game/level/sound managers)
Scripts/Menu
Scripts/
Sprites/GUI/
Sprites/
People have realised that you can cut down a little on recompile time by putting scripts inside the Plugins/ folder at the root. This skips one pass of compilation unless you change something in a script in there. The structure would be:
Plugins//
and
Plugins//Editor/
if you also have some custom editor panels.
With the asset bundle tools improving I think it’s worth investigating putting groups of data in those too, particularly graphics for different screen sizes.