which is the best nation to live a peaceful life?

Sweden, Switzerland, Denmark . . . there are several.

I agree with Set on the countries he mentioned, but I also think of New Zealand and Australia.

And I think the United States can also offer a pretty peaceful life. It's a big place so it depends on where you live, but if you keep out of politics, have a decent job and live in a nice community I think for the most part you can probably have a pretty peaceful life.

No guarantees anywhere. But try Scandinavia.