There are a few things that make the iPhone unique.
The first, and most evident is its operating system, iOS. Apple is the only smartphone that uses this operating system. That's because the iOs is a proprietary mobile operating system owned by the Apple company. The rest of the smartphones in the market, like those by Samsung, Sony, Google, LG, and several other brands uses the open-source Android platform. The result is that the iPhone is not dependent on any other company for software updates, etc.
Second, because of its unique operating system, one which was built specifically for the iPhone, it's hardware has also been tailored specifically for it, too. So, while the rest of the smartphones share the same chipsets, etc., Apple uses hardware that has been tweaked to match the needs of iPhone users, alone. As a result, the iPhone is more efficient than other smartphones.
Finally, the iPhone is unique because it is the only smartphone that can seamlessly work with the rest of the Apple products. You don't need to install anything if you want to sync your iPhone with your Macbook or iPad and vice versa. And it is also the only smartphone that allows you to take advantage of all the additional features of the Mac ecosystem.