The best way to rank up quick in gta 5 is to do missions obviously and the missions you want to do is the following:

- San Andreas Seoul
- Handle With Care
- High Priority Case
- Blow Up III
- Extradition
- Judging the Jury
- Rooftop rumble

Rooftop rumble is one of the best missions for RP but it unlocks after level 69, so do the other missions first.