Need for Speed: Hot Pursuit

Reach the top rank as a Racer. You will need 2,000,000 bounty to get to rank 20. Online and Offline modes apply to this rank. Getting all Gold Medals in the Career will get you to rank 12 or 14 and take about 20 hours.

For single player, you can replay any Hyper Series events you are best at for the best bounty rewards. I found the event "Spirit of Performance" the best for grinding out bounty because it's short and easy.

Here is an online boosting method found by Turkdaballa6:

Start a private match with 1 other person.
Player 1 will start out as a cop and Player 2 as a Racer.
Player 2 will stop at the start of the race and Player 1 will stop next to them to bust them fast.
The next round, Player 1 will be a racer and turn around and drive the opposite way as Player 2 so they escape to end the match quickly.
Switch side and let Player 2 win as each side.
Repeat to level up.
You should get around 8,000 bounty per match.