Global smartphone shipments hit 345 million units in the first quarter of 2018, a year-over-year decline of 2.4 percent. The numbers serve as further proof that smartphone demand is cooling amid lengthening upgrade cycles and slowing innovation.

Strategy Analytics says Apple's iPhone X was the best-selling smartphone in the world with an estimated 16 million units shipped during the first quarter. In fact, Apple dominated shipments across the chart as four of the top six most popular smartphone models were from the Cupertino-based company.

The other two devices in the top six, the Android-based Xiaomi Redmi 5A and the Samsung Galaxy S9+, had estimated shipments of 5.4 million and 5.3 million units, respectively. It's worth noting that Samsung's new flagships, the S9 and S9+, only started shipping towards the end of the quarter but are off to a good start. Strategy Analytics says it expects the S9+ to become the best-selling Android smartphone globally in the second quarter.

Apple shipped 12.5 million iPhone 8 models, 8.3 million iPhone 8 Plus units and 5.6 million iPhone 7 models during the first three months of 2018.

The top six most popular handsets collectively accounted for one in seven smartphones shipped during the quarter.