Each mainline GTA game did something genre-defining, making it instrumental to the success of the GTA franchise.
The GTA series is one of the most well-known and successful video game franchises of all time. When a player thinks of 3D open-world sandboxes, they often think of GTA. It's that genre-defining.
Even having crime be a central part of a video game often drives comparisons to the GTA series. So much so that "GTA Clones" are a legitimate term some people use to barrage games they view as inferior to the GTA series.
Both casuals and hardcore gamers adore the GTA series, so some people often wonder how this came to be. In fact, the GTA series wasn't always so universally popular. There was once a time where the GTA series was financially successful but seen as just decent games; not the revolutionary series as it's seen today. Predictably, this will only include the mainline games, as they're the most instrumental to the series (hence the moniker of genre-defining).
How each mainline GTA game did something genre-defining
#1 - GTA 1
As GTA 1 was the start of the franchise, it was understandably a critical moment in the the series' legacy. It laid the groundwork for players to have the ability to freely explore the world, kill NPCs, and do the occasional mission here and there. GTA 1 was seen from a top-down perspective, which would later return in GTA 2, Chinatown Wars, and GTA Advance. It's not as genre-defining as later GTA titles, but it is still crucial to the history of the GTA franchise.
#2 - GTA 2
GTA 2 helped introduce a more robust gang system, where a player's choices could influence their standing with one gang while hurting another. Each gang had their own distinctive looks, vehicles, and weaponry. Aside from that, GTA 2 has a more robust radio, allowing players to listen to some more diverse music compared to GTA 1. .
#3 - GTA III
From time to time, there's a moment where something monumental happens. While it may be a hyperbole to compare GTA III's legacy to a man landing on the moon, it is vital to know that GTA III permanently changed the GTA series for the better. It helped introduce the 3D open-world sandbox style that most modern GTA fans are familiar with. GTA III was so genre-defining that its Metacritic rating is 97/100.
#4 - GTA Vice City
As revolutionary as GTA III was, GTA Vice City was also instrumental in paving the way to the modern GTA identity. GTA Vice City's voice acting was miles ahead of GTA III in terms of star power and passion. One of GTA III's weakest aspects retroactively is that the characterization was dull and lifeless at times, lacking the GTA charm the later games had.
Fortunately, GTA Vice City helped make every character stand out memorably. Future genre-defining characters within the GTA series would definitely appreciate what GTA Vice City brought to them.
#5 - GTA San Andreas
Customization was surprisingly limited in prior GTA titles, which made GTA San Andreas's customization options all the more genre-defining. Players were able to change CJ's hairstyle, facial hair, tattoos, and clothing effortlessly, and that's not even mentioning how vehicles were customizable for the first time in a mainline GTA game.
GTA Online would later perfect some of the customization options, but it doesn't change the fact that GTA San Andreas was the first one to perform this genre-defining act.
#6 - GTA IV
GTA IV was the start of the HD Universe, where GTA games would take place in high-definition. The jump from regular 3D graphics to HD graphics was genre-defining, even if GTA IV's graphics aren't as impressive as other HD games. Aside from that jump, GTA IV's story is also arguably the best in the series, which helps make the game stand out even more in a lineup of top-tier video games.
#7 - GTA V
GTA V's online presence is legendary. However, GTA Online is not the only genre-defining aspect involving GTA V. Multiple protagonists with unique, distinct stories is something no other mainline GTA title did. Then there's also the matter of how large the GTA V map is, and how it dwarfs every other GTA map by comparison.
In fact, GTA V did so many genre-defining things that it's best to just summarize them as the reason that GTA V is one of the best-selling video games of all time.