This is determined not just by where a country happens to be on the map, but by which time zone its Government has chosen to use. These things can change over time, and it can be dangerous to believe what you read in the papers (or on their websites) ... as I shall attempt to explain.
The most advanced time zone on Earth is UTC+14, which is 14 hours ahead of Greenwich Mean Time. (UTC stands for Coordinated Universal Time.)
According to Wikipedia, only two countries currently use UTC+14: Samoa and Kiribati. Tonga (according to Wikipedia) used UTC+14:00 for Daylight Saving Time (DST) from 1999 to 2002 and from 2016 to 2017. DST applies at New Year in Tonga, when it's in force, so in those years – and in those years only – Tonga celebrated the New Year at the same time as Kiribati and Samoa. If Wikipedia is correct, Tonga doesn't currently have DST; it uses only UTC+13. Apart from DST in those years mentioned above (i.e. since at least 2018), Tonga has been, and is, an hour behind its neighbours.
Metro reported at 09:54 on New Year's Eve 2019 that Tonga, Samoa and Kiribati (/Christmas Island) would be first. Six minutes later – as 2020 actually arrived in the part of the world in question – a different reporter had pared down the list to Samoa and Kiribati. Maybe someone had put them right in the meantime?!
The Sun appears to have failed to recognise the error (if such it is), reporting that "The Pacific island of Tonga [would be] first to ring in the New Year[, celebrating] at 10am GMT on December 31 – making the tiny island nation the first to head into a fresh year." Further down the page, the article states (correctly) that Samoa and "Christmas Island/Kiribati" would be celebrating at 10:00 am GMT. I couldn't find a date on this piece, but it appears to have been published on or around 1 January 2020.
WorldAtlas said, in a piece dated 20 September 2018, that Tonga, Samoa, and Kiribati were first. But unless either Wikipedia or I (or both) have missed (or misunderstood) something, this was to become out of date not long after publication.
As for The Sun: by my reckoning it's a couple of years out of date – and so is this question.
© Macclesfield Quiz League 2020