Thanks for the comments.
Yeah the lawnmower derby is a bit lame. I think the most challenging is the 3 wheel super-van competing against the big rig. Also you can't unlock the big rig or complete the career without winning the challenge. I actually had to drop the difficulty level to get this done. It was a real chore.
The A class cars are a lot of fun but most of them unlock as you near the end of the career mode, leaving only online multiplayer to adequately try them out.
The weekly and monthly challenges are worth a look too, and award fame points that can be spent on extra ''exclusive'' vehicles and paint jobs.
Overall I think the most fun I had was the banger destruction races that actually had me laughing out loud for real

What was your least favorite event and most favorite type?

I didn't get as far as you but my least favorite was the lawnmower crash derby's.

Oh wow that's impressive!