Being able to project power outside its immediate region in both soft and hard ways.

Nukes exist for retaliation. They are a terrible first strike weapon. Having them also doesn't mean you can deploy them wherever. There are so much more important aspects to a military than those.

China has a major effect on the world economy, but it doesn't have a military to extend that influence. It's an economic superpower, but not a military one. Diplomatic options are improving, but far from the influence the US has.

Russia has a shit military and could previously dictate the fossil fuel economy in Europe, but not much else. It lost much of that economic influence while invading Ukraine (backroom deals continue, but it's not anywhere near what it was). The Soviets built the perfect counter to the US Navy by focusing on submarines, but those old subs are badly outdated. It's not a superpower at all.

The EU (taken as a whole) also has lots of economic and diplomatic influence. None of its constituent powers has much ability to project military power, though. UK and France both have aircraft carriers, but neither country can solve two problems at the same time with its military, and may even struggle with just one problem if they're on their own.

The US has both a military that can project worldwide, plus economic and diplomatic power to go along with it. Its collection of carriers can solve five problems at the same time and still have some left in reserve. A major multilateral world treaty without the United States would be considered incomplete. That's a superpower in a complete package.