My vote goes to Syscoin. I think they have one of the most secure wallet out there. They definitely have a hardworking team and I believe masternodes will be coming soon.
Syscoin has a good decentralized marketplace, but it lacks adoption unlike others like OpenBazaar. Perhaps, if Syscoin could increase its awareness by promoting itself around every corner, then it could become a leading altcoin in the market. The implementation of masternodes, if real, could become very attractive for Syscoin as it helps boost its demand at a fast pace.
Despite this, I believe that more decentralized marketplaces would come up in the future, as people realize their true potential. OpenBazaar is a good and widely used marketplace, but others like Particl, and Syscoin are also good and they achieve the same purpose as OpenBazaar. It would be only a matter of adoption, to determine which decentralized marketplace would be the "best" in the mainstream world.
Nevertheless, if there happens to be new decentralized marketplaces, I would like to know about them.