As said application will help dictate what you might want to use...
For me I use the following Bondo Spot Putty, Squadron Putty (thin with acetone), Bondo (thin with polyester resin, acetone or styrene **FYI styrene is a clear liquid solvent, not to be confused with polystyrene**) I have also done the talc or corn starch and super glue, as well as heavy bodied epoxies like JB weld, or even 5 minute epoxy... Heck I have even used a pour of casting resin for larger fills... For surface pin holes or scratches, even small seam lines, I generally just brush on a coat or two of 2K urethane primer in those areas and sand smooth...
Lots of options depending on what you are doing...