Hello and then. It's nice you have opened a thread on this, and hopefully you'll get plenty of useful replies.
Let me tell you one thing first: Just because I do not see the USA (i do not like your country being called 'America', as I find this pretty ignorant towards the other American countries) as the most powerful or greatest (what does 'greatest' even mean in this context? It's very polemic.) nation there ever was, it does not mean I find your country inept, or even evil. I just do not see it that relevant in a world historic context. There were many nations who accomplished far more in very different times, without the mobility available now. There was the Roman Empire, the Mongolian and Islamic Expansion, Alexander's realm, the Ottomans, or the Holy Roman Empire, just to name a few.
As for what other nation should take the USA's spot as THE superpower, which I don't even think you are anymore, as China or the EU, if you see it as a whole (not being as militaristic as the US does not mean being weaker), are very much equal players, I would choose none. A balance of power, i.e. several countries on different continents 'sharing' the power, would be best in my opinion. I do not need one nation forcing their views on th rest, as it usually is, if there is one nation that is by far the strongest. A balance could be a benefit for all of us.

And there are quite a few countries on the rise right now. Just have a look at Brazil!
I hope you could follow me. I typed this on my Phone. Kinda hard to keep track!