The many platforms that conform the term 'Arcade' has got to be considered the most extensively researched and emulated platforms in existence. Nintendo 64 Project64 v1.7 Playstation 1 Mednafen

Genesis core is experimental but mostly functional 2 Players only.

While it only emulates Genesis/Megadrive, Sega CD and 32x, it uses Kaillera for online multiplayer, which is helpful in case you get consistent desynchs while using KegaFusion. It emulates practically every single console and peripheral prior to the Saturn, all while allowing for some decent Netplay capabilities on the side.

Concerning Sega emulators, this is really the only one worthwhile option to netplay with.

I'm not sure if it's possible to get it to work.

Still shows signs of being updated, but it absolutely HATES anything other than stock Kaillera.

The author did away with Netplay in more recent versions. Has a very nice interface but the documentation doesn't indicate what ports it uses, so you're limited to using a VPN like hamachi. It will also attempt to automatically add port-forwarding rules on your router via UPnP.

Mesen supports an easy-to-use NetPlay feature that always stays in sync.

bsnes, FCEU, QuickNES, or Nestopia cores citation.

To do that, follow the Port Fowarding Guide For Netplay. Before you get started, you should note that some emulators and clients will require you to forward your ports.

Mac snes emulator with netplay