Which economy will be better in the long run...USA, Japan or Europe ?

Japan as it is learning to use its neighbours. Before WW2 they were the super power.

Here is a surprise answer - China

Not in the near future, but after say, three decades. Three pertinent reasons

High growth sustained over last 15 years

Work ethics and culture

Success in Population control

The two weak points viz. politics and closed economy are being appropriately addressed by them - they do have a burning desire to excel, not just individually but also as a nation.

America

Unfortunately, America's time has come and is going. It was a hell of a ride, but we just got to smart for our own good. In fact, we committed the basic sins and undoings of the Democratic form of government, we got complacent, arrogant, ignorant and selfish.

The general public got complacent. The ruling class got arrogant. The ignorant did not vote. And, our society is just plain old selfish.

To bad folks, it could have been beautiful.

The most powerful one is USA.

However, the economy that is growing at the fastest rate is actually China. It is projected that by 2030 China's economy could be as powerful as USA's.

Why you didn't mention Asia? In the long run, it's going to be Asia, excluding Japan as current technologies, most Asia Countries are almost in par with Japan, if Japan still refuse to open up (insist to use Japanese as their official language), Japan will lose out to most Asia Countries in the long run.

i think in the long run, it won't be different economies but one global economy..

if the US experiences a recession in their economy, they always go to war anyway to bring the economic boom back.. just check out the history on economics of war

So far, i have no clue! Because any economy that woult try to break economical/dependance links whit the US would suffer a boycott

usa