Often Americans will say America is the greatest.
Canadians will say Canada is the greatest.
French will say France is the greatest.
And so on. It's not hard to see why people think their own country is the greatest, they've been indoctrinated into it, or they've just grown up there and are comfortable with it, or they don't know anywhere else... or maybe other reasons.
But, if you're going to give a UNIVERSAL assessment of what makes a country great, what would those things be?
I'll tell you what doesn't make a country great... One man. That kind of shits all over MAGA. Fucking moronic slogan. It is not the president or leader of a country that makes it great.
As to your OP, it's purely subjective. To my mind, it's the people of a country that makes it great. In the case of the United States, I'd say the people and the Constitution. Not the President, not the congress...
MAGA was Trump asking for support so that WE could make America great again.
From his presidential announcement. It is only 18 seconds. Watch it.