I'm wondering for new players what would be a good starting server?
It doesnt matter much. Your world is only an option when you play World vs World (a type of massive player vs player). For regular PvP (5 vs 5) and open world PvE, the choice has no consequences. Just make sure you pick EU when you play in EU or similar timezones or NA when you play in Asia, Oceanic, or north and south America.
Go for the most populated server, maybe. Because if you want to move into a popular server (after your first free transfer) it costs more gems to get into a higher population.
You can't tell which is the most populated server, only which server(s) have the most WvW players.