I got to level 20 with Brown Thunder. I could have kept going, but after level 21 started I'd had enough and cancelled it. Then went back to the Hyman Condos hideout to work on reducing my wanted level. (It's easy to avoid the police by just going to the roof or ground level when they come. The police bribe there also regenerates). The police tend to get irate with you flying around blowing everything up. Anyway, I think the level 20 award was $20K. Maybe $200K. I have about $2.5 million total.
It was interesting this time. I started around the airport. At one point it put some criminals on the bridge to the first island, then put everyone else over there. That one's a bit tougher since there're more things to hit.
Also, after level 12, the cars split up. It's still not too hard to get them though; it just takes longer.
I have had the game freeze on me twice while doing that mission, though.
It's also the easiest way to get the level 12 vigilante mission done. I think so, anyway. Just don't blow yourself up by being too close to the explosion or firing rockets while turning hard and hitting yourself. And, if you bounce around and start to lose control, press and hold down the R3 button (the horn, usually) while holding x, and it'll level itself out into a hover. That works with all helicopters.