What is the best dermal filler for box car acne scars?

Your question presupposes that fillers are necessarily the ideal treatment for box car acne scars. While longer-lasting fillers, like Radiesse and Restalyft in small amounts (my choices in my Upper East Side Manhattan practice) can be helpful for elevating depressed scars, more often than not the central depressions of these kinds of scars are so fibrotically bound down that the filler cannot lift it up sufficiently; instead, there is a risk that the filler can spread to the sides and around the bound-down portions of the scars and result in something of a donut type appearance (with the surrounding area being elevated above the hole giving rise to this unwanted result). (In my Israel satellite facility where a far greater number of regulatory agency approved fillers and volumizers are available, I typically opt for Sub Q or Hyaluronica 2 for this purpose).In most cases, donut hole problem can be avoided, if subcision, a quick, simple, and inexpensive procedure is performed before the addition of the filler, either immediately before or about a month before the filler is injected. For more information on subcision and on other possible acne scar treatments, check out the archives of Realself.com. And be sure to consult with a board certified aesthetic physician with experience and expertise in all facets of acne scar treatment. 

Its not the filler it is the doctor doing the treatment that is the best determinant of your cosmetic results. Seek an experienced, qualified doctor and after an adequate examination of the extent, type and severity of your scars, the doctor will make recommendations for your treatment. 

Filler works well for acne scarring, but the injector and the filler matter. If your injector isn't skilled in injecting for scarring, you risk bad results as this is much different than filler for volumizing. Most often I use Restylane, Juvederm Ultra Plus, or sometimes Radiesse. "This answer has been solicited without seeing this patient and cannot
be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."