Depends on a few things but everything is cross server except mythic raiding during the raids current release
Faction: some servers are more dominated by a certain faction
Raiding: some servers have higher caliber raiders
PvP: Better to find a server that is balanced server wide for faster ques unless you want open world domination lol
Flipping: depends on what you want to flip
RP: if thats your jam just go to moonguard or however you spell it weird ass server