I have just been playing online normally so have naturally ranked up to around level 50 or so through Heists, Missions and Races etc.
Since the CEO work has been released I find this the best way to rank up quickly. On Average a Crate buying mission rewards around 2500RP and a Crate selling mission rewards around 5600RP.
If you work as an associate for a CEO and you stay in their vicinity you also get 200RP every minute or so.
If you are looking for something different and fun and that will massively help you level up then doing CEO work is currently the best way in my opinion.