1.8-1.8.9, as it's the actual version of the game but you can use versions above it aswell, I personally use 1.12.1 as it's easy and supports generally majority of the servers.
I generally find 1.12 the best to use on most of the server (though mostly because it's the only version I have Optifine on). Any version from 1.8-1.12 will work in the game, I think 1.13 may work but don't take my word on that.
