What's the best version for faction server?

I'm trying to make a minecraft server (and a gmod server) but I'm wondering is there a gamemode that's best for survival/factions? some people say 1.8 and 1.14

I would say 1.8, as they completely screwed the combat in 1.9. You can use the plugin ViaVersion so people using 1.9-1.14 clients can join.

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If you use ViaVersion read through the config file, some settings are there to remove bugs.