An AWD Vulcan with rally tires and turbo swap can stay under 998, grab wicked air, get you from area to area easily, and looks the part if you put RAF roundels on the sides.
if you have all cars and all dlcs then: agera rs, mosler, shelby king cobra, 599xx evo
if you have only standard game and no dlc, only autoshow cars, then try aventador 2012 or cars similar to lotus eleven or jag d-type
I know this is a kind of an old post, but I am going to post this just in case anyone is still coming here for guidance (like I was). I have found that the local motors truck with all upgrades, lowest pressure tires, soft springs, and a good gear tune, lifted as high as possible, works best for most offroad jumps.
In the last couple of days I have managed to rise to the top 1% on several of the danger zones using the Mosler. I have been able to exceed the 3 star distance by 300 feet or so by using this car and finding the right path.