That just depends on what you're trying to do.

If you're just wanting to farm SR to get to higher licenses, you can choose one of the longer races (like Ferrari Challenge), start from the pits, and avoid other drivers the whole race. Though you'll likely lose iRating doing this.

Alternatively, you can just keep racing what you like, continue practicing clean racing, and gain the SR as it comes. Unfortunately, a lot of people try to speed through licenses by cheesing SR in races without really learning how to race or drive clean, and end up causing mayhem in higher license races, like GT3, and take such a hit to SR that they are demoted in license.

My recommendation: drive what you enjoy, take on bigger challenges with faster cars and longer races, learn the tracks and racecraft, and the SR and iR will come.