As 2017 hits the halfway mark, it seems apt to draw breath and look back.

The way we assess the critical and commercial impact of a pop song regularly shifts. For instance, as CD sales have become even more anaemic and streaming services have now usurped download sales, most of pop's biggest names must now live with puny sales figures.

Yet thanks to the enduring appeal of radio and the colossal subscriber numbers generated by streaming services Spotify and Apple Music, the sheer and utter ubiquity of a big hit remains.

From an artistic perspective, Lorde's superb comeback album Melodrama is comfortably the best record released hitherto.

In contrast, Katy Perry's album Witness was a circumspect flop. Within industry circles, doubts quietly shroud the viability of Perry's tenure on pop's A-list.

Perry's slip must seem especially galling to her when she ponders how the dull and apathetic duo Chainsmokers now pass as a global superstar act. Equally depressing: the uninspiring Imagine Dragons are currently rock's most likely commercial breakout.

It's not all bad, though. The eminently affable British singer-songwriter Ed Sheeran is excelling at every conceivable metric for success in 2017, while Queens of the Stone Age have released their first new music since 2013.

Drake is transcending pop and hip-hop with a seemingly effortless but extraordinarily prolific level of efficiency. His single Passionfruit is this year's best so far. Yet another superb new dancehall single, Signs, has just arrived, launched at a Louis Vuitton show in Paris.

Some of hip-hop's biggest stars have been remarkably productive thus far. Kendrick Lamar surprised with the caps-heavy album DAMN. Future released two albums: the heartbroken HNDRXX and a more boastful self-titled effort. Both topped the US chart within consecutive weeks.

Likewise Chance the Rapper is headlining stadiums globally as perhaps the world's most successful true indie artist. Jay-Z is back playing stadiums too, with a new album released exclusively on Tidal, a platform many of his fans despise.

Then there's Calvin Harris who in February boldly proclaimed (via Twitter) that he had "worked with the greatest artists of our generation!!!" and backed up his assertion with four excellent singles and a very solid new album.

The most popular pop song of the year so far, Despacito, is sung predominantly in Spanish. There have been further anomalies. For one, veteran DJ Khalid is enjoying a rare purple patch.

Gorillaz released a new album that failed to generate a hit; Linkin Park baffled their once significant fan base with an almost unrecognisable new sound.

Selena Gomez had a David Byrne-approved hit with a song based around the key riff to Talking Heads' Psycho Killer.

Former One Direction members Niall Horan and Harry Styles channelled the Eagles and Harry Nilsson, respectively.

Miley Cyrus abandoned her prodigious pot smoking habit and unleashed Malibu, a throwback to Fleetwood Mac circa 1975.

There's been no shortage of female voices in pop: Dua Lipa​, Alessia Cara, Kiiara and Julia Michaels headlined a new breed of youthful pop stars.

In rock circles, a cluster of acts including the National, Muse, LCD Soundsystem, the Killers, Grizzly Bear, Broken Social Scene, Paul Kelly and HAIM dropped new music.

A trove of new Australian acts continued their development. Melbourne's Husky and Perth's Methyl Ethel impressed with their full-length releases.

And look for Alex the Astronaut, Winston Surfshirt, Middle Kids, Rolling Blackouts Coastal Fever, Amy Shark, City Calm Down, Camp Cope and Gang of Youths to add to the excellent music they dropped in the first half of 2017.

For now though, here are 50 of the most notable tracks of this year.