GTA 5 and GTA Online might be two sides of the same game, yet there are notable differences to consider. Some players may prefer the former game, while others like the latter more. This article will focus on the single-player title and some reasons why it can be a more fun game than its multiplayer counterpart. Your mileage may vary.
Generally speaking, GTA 5 is the better game for people who love traditional Grand Theft Auto games. It flows well and has hundreds of hours worth of content for you to enjoy. GTA Online also has a plethora of content, but the multiplayer aspect might not be for everybody.
Note: Some aspects of this article are subjective.
Riveting characters, mod support, and more ways GTA 5 is a better game than GTA Online
1) Better characters and story

Even if GTA 5's story can be a bit divisive compared to past titles, it's still more interesting than what transpires in GTA Online. The former title takes players through Michael, Trevor, and Franklin's intertwined plots. While some characters are forgettable (i.e., the antagonists), much of the supporting cast is amazing, such as Lamar, Jimmy, and Chop.
GTA Online has some eccentric characters like Agatha Baker, but it shares much of the same cast as its single-player counterpart. As far as the storyline goes, there are a few interesting plot points, like The Doomsday Heist scenario, but much of it is rather run-of-the-mill in terms of narrative.
Some players will find the tension between Michael and Trevor in the single-player game much more enjoyable.
2) Less repetitive game design

GTA 5 is like a standard Grand Theft Auto game where you have missions that advance the main storyline. Alternatively, you could travel through the world and experience fresh Random Events or Strangers and Freaks. You will rarely encounter the same activity multiple times.
By comparison, GTA Online's core gameplay is much more repetitive. Here, the main driving force for some players is to grind for money. You can be wading through the same heists and Sell Missions ad nauseam. Executing The Cayo Perico Heist a few dozen times gets tedious overtime.
3) Significantly better balance

Multiplayer games tend to run into balance issues, and GTA Online has no shortage of flaws in that department. Apart from grinds feeling repetitive, the game has terrible vehicle balance. HSW cars far outpace non-HSW automobiles, while Flying weaponized vehicles tend to reign supreme, making hundreds of cars feel obsolete.
GTA 5's balance feels much better. For starters, a single-player game ensures that you won't be running into griefers or try-hards. That means it's just the NPCs you have to worry about. The AI here isn't teeming with aimbots, which can make this game feel more fair to play.
Not to mention, the vehicle balance is much better since this game never got the last several years of additions from its multiplayer counterpart.
4) Mod support

Mod support helps keep GTA 5 feeling fresh, even if you've finished practically everything else in the game. There is no shortage of mods that can appeal to your interests. Everything from changing character models to having new scripts is possible. For example, if you wanted to play as Tifa Lockhart in Los Santos, that option is available.
By comparison, GTA Online's mod support is much more limited by virtue of not being supported by Rockstar Games. It violates the game's terms of service, which can lead to a player's account getting banned.
On a related note, some players may love GTA RP, which is a modified form of GTA 5.
5) Single-player will always be playable

One unfortunate foible that nearly all online multiplayer games are susceptible to is the eventual end of their servers. Even GTA Online has suffered from this before, as their PS3 and Xbox 360 servers are no longer up, meaning players on those consoles can never dive into them via normal means.
By comparison, GTA 5 will always be playable several years to decades from now. Players never have to worry about losing access to their account or the countless hours put into their progress. It might not seem like a big deal now with the popularity of the current and next-gen ports, but this is something to consider down the line.
These were just five reasons why some gamers might prefer GTA 5 over GTA Online. There are other valid motivations for this topic, so let us know if you enjoy the single-player experience over its multiplayer counterpart.