FUEL

I got this right at the end of the game (as the name suggests). I think you may get it without having to do the multiplayer stuff - i.e. complete all challenges & races to 3 stars & find everything on the map, but I had already done the 3 multiplayer achievements so when I finally finished my last challenge this & obsessional challenger were all I had left & both 'chieves popped together.

EDIT: This is the equivalent of a platinum trophy on PS3 & will pop when all other 'chieves have been 'chieved, including the online race ones, although you will still need to find 74 liveries (or 79 if all bonus codes used) after getting obsessional painter as that 'chieve is slightly glitched (in a good way) & pops earlier than it's s'posed to.