Fully maxed out Arena Contender Issi or fully maxed out Elegy Retro Custom. Issi is definitely the best bet currently though as a duped version will sell anywhere from $1.7-1.9 million per dupe. Whereas the Retros only sell for about 900k, give or take.
Keep in mind that the sale price always goes off the cost of the customization options, not the purchase cost of the vehicle. So whatever has the most expensive and highest amount of customization options is what will sell for the most money.