If there was the best one, all others would have ceased to exist – everyone would choose the best, abandoning the others.
It all depends on your needs, selling style and efforts you invest;
what do you want, to sell at fixed price, to allow buyers to offer and negotiate, to open the auction, and then various types of auction (with or without reserve price, with or without buy-it-now option, dutch auction, etc);
are you willing to allow buyer to pay in monthly or even yearly installments;
do you want escrow service or you are comfortable with PayPal or bank transfer or some other direct payment;
who is your target, resellers or end users;
do you want to advertise your domains and send mails or calls to potential buyers, or you just want to list them and wait for buyers to find it themselves;
are you willing to point your domain’s DNS to the marketplace or prefer keeping your own DNS and just listing on the marketplace…
etc, there are various options that make a place the best for you and worst for me.
The best one for me is dan.com (previously undeveloped.com). Apart from Flippa and DAN, you can try a dozen of the others, such as Sedo, NamePros, Afternic, Uniregistry, Snapnames, and marketplaces at known registrars – Dynadot, Epik, Namesilo, NameCheap; if you have brandable names, BrandPa and BrandBucket are your destinations.