Melting pot. Then, all come together striving for common goals for themselves, their children, their country. Otherwise, you end up with all separate communities trying to push their own agenda, and no cohesivness. Look at the gangs, and how they embrace diversity.
With diversity you will need separate laws for each group, not all groups have the same beliefs and laws. You cant have one group be held accountable and another not, because of cultural differences. With diversity someone will always be offended if another comes into their area and doesn't act according to what is culturally acceptable to them. Wars are started because of that.
There is no way to manage a country from a thousand different perspectives. It can't be done. A country has to be untied to succeed.
Germany, Britain, and France have all said multiculturalism is a failure. Of course no expects people to give up their heritage, but I it's best to celebrate it, not flaunt it.