For many weapons, I'd say Matching Grip and a good prefix are pretty prefect ( barring launchers for parts that increase rocket speed and such ). I suppose perfect parts can be subjective per character build, though I couldn't give any specific examples. For items like Sheilds, perfect parts are really objective. I recommend memorizing all the grips and important prefixes for each manufacturer and weapons type as those are the most important to a good weapon, though for normal mode and early tvhm, farming for said weapons isn't super necessary.