FWIW, I got to 150 in a large part by running tons of Rivals challenges. It also helped my level up for free parts, and gave me a huge bankroll.
Besides all that, it's been a blast in general. It's a kick to go up against you guys in the OSS.
Also, you can wind up with some interesting matchups against others. I forget what challenge it was, but the other day I was running one on Tzukuba, and wound up against a Dart. This guy was all over the place, sliding, drifting, etc., but on the straight he would just blow by me. It took me a while, but I was determined to beat the guy (it became a matter of pride and principle; running that glitched monster on that track).
When I finally did (it took me a couple dozen laps), I was taking a bow, thanking the crowd, etc.

I guess you had to be there.

Chuck