From what I've looked at, literally any hour at least one person is on, it doesn't matter what server (unless its password protected), normally I see the most people at 1-6 AM Japan Time. However its different for what day you play.
It should be noted at around 12:00 PM at British Time (and all other European Times) practically nobody is online. This is because most people are asleep at this time. Because of this, i don't see too many players online :/