Forza Horizon

If you take the fast travel to the media center right above the Montano Plains and go east a little and you'll find a huge golf course, spent maybe 30 min and got from 98 to 1 in just a lambo mainly doing drift circles, plenty of jumps and space to drift to chain huge combos, just dont flip or hit the small patches of trees

This is a pretty simple achievement if you just play the game and focus on everything else first. But if you really want to knock it out as fast as possible then I advise practicing your skill strings. If you can get up to 5k or more points with a 2-5 times multiplier consistently then you are set. The easiest places for you to get high strings set up are places like the Golf Course where it is nothing but open range for you to continuously get air combos and drifts. There are places like the golf course here and there with different terrains, these make out to be the best places for drifts and air combos. If you want near misses and speed combos then I-70 on the west side of the map is the way to go, you can get huge points and multipliers going on here because of all the cars and the straight runs.

Edit: Thanks to Virtual Boy for this one. If you have the money and don't mind spending it you can purchase a multiplier boost with tokens (these can be purchased with microsoft points). Just go to the pause menu, then sponsorship challenges. You will see an option to hit the right trigger to purchase a boost. I also found that the best way to rack up really big points is to purchase the boost, set the game to easy and go race in the street races. Stay at the back for a while drafting and drifting. Then start passing everyone, weaving in and out of the AI opponents and getting near misses with the traffic. Once you have passed everyone hang back until they pass you again and repeat. I could consistently gain over 10-12 levels every race doing this.

Edit: Thanks to Old Beann for this addition. Start where your garage is, just string drifts together in the circle, I was getting 150,000-200,000 total points each combo, with each combo taking no more than a couple of minutes.

I found a good spot to do some quick leveling...Near Gladstone Drive there's a small curvy section of track. Within that, there's a tight bend that points to the northwest. I ended up there after a race (BMW Battle). I was using a Lambo Aventador, though any high-powered RWD will work fine.

I just swept from side to side, doing e-drifts around the corner, pulling the e-brake to spin around on each side, as the road is kinda wide. I'd usually get a great burnout and ultimate 180 skill, then do it all over again. Think I pulled a max combo of near 80k. There's occasional traffic, but not much - the only thing you really need to watch are the festival racers, as they'll plow right into you.

I used the interstate section on the western side of the map. It stretches to the very top to the bottom of the map. I used a fully upgraded RX-7 but you can use anything that goes over 200+ mph. All you have to do is drive up and down the interstate getting Ultimate Speed (1000), Daredevil (1000), Threading the Needle (1000), Awesome Near Miss(es) (500).If you use 2 tokens to get double the popularity. You can get it less than an hour if you can read the traffic and maneuver a car pretty good.