This depends on your game, if it is cross platform etc. For example gamecenter has some limited turn based multiplayer functionality. However for a full multi player experience you will need a server if the game is complex (in terms of what is being communicated and to how many people it is being communicated too).
Generally you are going to want to use a server when you get to that number of players anyway. Gamecenter isn't designed to handle large scale multiplayer. It is designed for a few people to connect devices and then shovel the networking information to every client. This is all good for a small number of devices but as it grows the amount of data increases quickly to unmanageable without a server to distribute it in an intelligent way.
I have heard good things about photon https://www.exitgames.com/en/OnPremise But normally best would be a subjective thing.
Photon Cloud is great, easy to use and works with Unity3D. I'm using it for my air combat game: http://steamcommunity.com/sharedfiles/filedetails/?id=244198767 Cheers!