Best Version of Minecraft Server?

If you ask for my opinion, I would recommend either 1.8.8 (Use the paper jar or a fork of paper) or 1.12.2 (Same thing, use the paper.jar or a paper fork). These versions have the best server performance and most (if not all) major servers use it. If you aren't running a server with low ram (we're talking 8+ gb ram) and you arent expecting too many players, then sure, go ahead and use post 1.12 versions. There are tradeoffs for both of them. It really depends on what you want.