Q:

What's the fastest way to level up in GTA online?

What's the fastest way to level up in GTA online?
I want't to level as fast as possible to beat my friends. But what is the fastest way to level up?

A:

The best way to level up is easily to perform jobs.  Rockstar patched the ability to replay missions, so I would recommend doing either races or survival.
Try out this quick race for farming levels:
Settings 

Type: Race
Laps: 1
Map: Criminal Records
Vehicle: motorcycles

Time to complete this takes about 30 seconds and you will get about 800 XP.  
Each wave of Survival you will earn about 1000 RP per wave when you play alone.

A:

Something I like to do to rank up fast is to have sex with hookers.
You get 1000 RP on each encounter.

A:

If you go into a Last Team Standing mission with some friends and while on the team select screen, have everybody join one team. The mission starts and ends within seconds saying that you all won. You can get a couple thousand dollars and about 900-1000 RP for it. Including loading times, you should get 900-1000 RP every 60 seconds or so.
This is a much faster way of leveling than the previous method I used which was to repeat the Criminal Records race over and over. 
(By the way, the Last Team Standing mission that I discovered this on was Cypress Flats. Don't know if it works with every LTS but I would assume that it does.)

