Based around a viral dance craze, the video sees the star rapping on Westminster Bridge and features a cameo from Idris Elba.
Sam Smith and Normani's Dancing With A Stranger was the second-biggest video. Billie Eilish's Bad Guy came third.
Outside the UK, Latin music dominated, accounting for all of YouTube's top five music videos.
Top of the list is Daddy Yankee's Con Calma - which interpolates Snow's 1990s rap classic Informer; while electro-flamenco singer Rosalía takes second place with the earworm groove of Con Altura.
It's the third year in a row that a Latin song has topped YouTube's global chart. In 2018, the honour went to Nio García's Te Boté; while in 2017, it was Luis Fonsi and Daddy Yankee's Despacito (which is the most-viewed video in the site's history, with 6.5bn views).
The figures reflect Latin America's increasing importance to the music industry: It's the world's fastest-growing music market - revenues increased by 16.8% last year, with Brazil the largest contributor.
"Without any doubt, there was always huge music consumption in Brazil - it's a country that truly has music at its heart - but, due to copyright infringement, this was never recognised," said Afo Verde, chairman of Sony Music Latin Iberia, earlier this year.
"When consumption began to take the form of streaming, the results were impressive."
"I think it has been a sleeping giant for years and years, and the reality is that it's a country with more than 200 million people. And it's a country that loves music," added Jesus López, chairman of Universal Music Latin America.
However, the spectre of piracy still looms over the market. It's notable that Latin artists thrive on YouTube, an advertising-supported platform, but have a lesser impact on services like Spotify and Apple, which generate larger revenues through monthly subscriptions.
Both Spotify and Apple Music released their own most-streamed charts this week, and artists like Daddy Yankee were conspicuously absent; with Western stars like Camila Cabello, Lil Nas X and Post Malone populating the countdown.
You can compare the various charts below:
Most-streamed songs of 2019 (UK)
Spotify Apple Music YouTube
1) Someone You Loved - Lewis Capaldi 1) Someone You Loved - Lewis Capaldi 1) Vossi Bop - Stormzy
2) Bad Guy - Billie Eilish 2) Old Town Road - Lil Nas X 2) Dancing With A Stranger - Sam Smith & Normani
3) I Don't Care - Ed Sheeran & Justin Bieber 3) Thank U, Next - Ariana Grande 3) Bad Guy - Billie Eilish
4) Vossi Bop - Stormzy 4) Location - Dave ft Burna Boy 4) 7 Rings - Ariana Grande
5) 7 Rings - Ariana Grande 5) Sweet But Psycho - Ava Max 5) Old Town Road - Lil Nas X
Most-streamed songs of 2019 (Global)
Spotify Apple Music YouTube
1) Senorita - Camila Cabello & Shawn Mendes 1) Old Town Road - Lil Nas X 1) Con Calma - Daddy Yankee & Snow
2) Bad Guy - Billie Eilish 2) Sunflower - Post Malone & Swae Lee 2) Con Altura - Rosalia & J Balvin
3) Sunflower - Post Malone & Swae Lee 3) Thank U, Next - Ariana Grande 3) Secreto - Anuel AA, Karol G
4) 7 Rings - Ariana Grande 4) 7 Rings - Ariana Grande 4) China - Anuel AA, Daddy Yankee, Karol G
5) Old Town Road - Lil Nas X 5) Bad Guy - Billie Eilish 5) No Me Concoce - Jhay Cortez, J Balvin, Bad Bunny
Apple also released data about the year's most searched-for songs on the music discovery app Shazam - with Lewis Capaldi's Someone You Loved coming out on top.
Meanwhile, Joel Corry's sad-banger Sorry broke the record for the most Shazams in a single day. A total of 41,000 people looked the song up after it featured in an episode of ITV 2's Love Island on 24 July.
"My phone went into meltdown when it happened," the producer said at the time. "It's got everyone talking, and helped launch the single up the Official Charts. I'm just buzzing everyone is loving Sorry!"
Most-Shazammed songs of 2019
1) Someone You Loved - Lewis Capaldi 1) Bad Guy - Billie Eilish
2) Piece Of Your Heart - Meduza ft. Goodboys 2) Someone You Loved - Lewis Capaldi
3) Dance Monkey - Tones And I 3) Dance Monkey - Tones And I
4) Giant - Calvin Harris & Rag 'N' Bone Man 4) Calma (Remix) - Pedro Capó & Farruko
5) Sorry - Joel Corry 5) Piece Of Your Heart - Meduza ft. Goodboys