With Gameloft increasing the cost for cars with each update, which car has been the hardest for you to reach MAX PRO? I'll start with mine being both SSC cars with their massive MAX cost along with their pro kit formulas. It doesn't help that both cars are now usless in MP against the S and A class kings.
Other dishonorable mentions
Mastery Cars: Will eat through your stock of engine cads. Especially so with the 280Z and C16 with their ultra rare custom engine cards.
Bugatti Veyron, VW W12, Mazda Furai & Ferrari Testarossa: Will only accept custom engine cards that are rare to come by in random boxes and only given out in plenty if you're at the top of the global leaderboards in their apex wednesday cup.