It's strange to think of memorable stories occurring outside of the main storyline, but the GTA series does have several side stories worth mentioning.
GTA's main storyline is usually pretty solid, yet it's often the subplots that end up overlooked. In some cases, the subplot is better than the main story, while it's a fun diversion in other situations. Either way, GTA's best side stories are memorable and make the player want to see more. Of course, a subplot can still occur in a primary storyline mission, but it doesn't necessarily have to.
Considering many of GTA Online's missions don't really follow a defined path, those missions won't be included (especially since GTA Online isn't finished with updates yet). All GTA games have memorable stories, but the more popular and widely talked about titles will have precedence for the sake of what defines a story as "memorable."
For example, Liberty City Stories has some great plots, but most players haven't played it, making it hard to rank it as "memorable."
Five most memorable side stories in GTA (outside of the main story)
#5 - Dr. Friedlander Missions (GTA 5)
Dr. Friedlander is the prime example of a useless doctor. He has no genuine concern for his clients; his primary objective is to earn easy money. He listens to Michael's concerns during therapy in a bored manner, only caring to ask about his sexual experiences or dismiss Michael from his office.
His manipulative behavior doesn't endear him to the GTA community, but it is nonetheless memorable for how disgusting he is to Michael and his troubles.
He may have once been highly respected for his line of work, but it's clear through several of his GTA 5 missions that he's just in it for the money. Players can see how the doctor just waits for the sessions to end, not even blinking at the thought of Michael confessing to several brutal murders.
It is GTA, so murders are commonplace, yet fans would assume that these memorable missions would dive into the issue more.
#4 - InterGlobal Film Studios (GTA Vice City)
While players need to purchase and complete several business-associated missions to trigger the final mission in-game, they don't actually have to own or do all of them. As long as Print Works is one of the completed properties, players only have to do five other ones.
Given that InterGlobal Film Studios is the most perverted option for an asset, some players might remember how questionable some scenes were. This is especially painful for players who remember their parents walking in on any of Candy Suxxx's scenes.
Some of the missions are fun, but they were memorable mostly because of how inappropriate they were.
#3 - Zero's RC Missions (GTA San Andreas)
Zero's RC Missions are memorable for two reasons. First is that the premise of these missions is childish and silly, especially since they deal with RC vehicles and grown men finishing their rivalry with a weird board game. The second reason is how frustrating "Supply Lines..." is as a mission.
RC Vehicles tend to be awkward to use, but adding a time limit is always a recipe for disaster. While "Demolition Man" from GTA Vice City was mandatory, "Supply Lines..." wasn't, which led to some players never even bothering to finish it.
This story's silliness is charming, but the difficulty in the clumsy controls is memorable for the wrong reasons.
#2 - Lester's Assassinations (GTA 5)
Only the first assassination is required for the main storyline; everything else is optional.
Players seeking to make the most money in GTA 5 will love Lester's Assassinations, as they are allowed to manipulate the stock market to make an enormous profit. It doesn't affect all stocks, but specific missions do alter the performance of some stocks.
If efficiency were the norm for defining a subplot as memorable, then Lester's Assassinations would be the most memorable story in the game. Unfortunately, the story's main plot is rather dull, making it rank lower than the next entry on this list.
#1 - Breaking the Bank at Caligula's (GTA San Andreas)
The series of Triad missions involving a heist at Caligula's is easily the most memorable subplot in GTA San Andreas (and arguably in the GTA series).
It isn't relevant to CJ's quest to get revenge on Officer Tenpenny or Big Smoke, and he doesn't actually need the money to do it either. Rather, these series of missions are goofy yet highly memorable in their execution.
This subplot includes six missions, with the two most notable ones being where CJ can date Millie Perkins and the actual heist itself. Gimp suits are iconic, but the actual heist itself is epic.
For some fans, it's the most fun mission in all of GTA San Andreas. Given how popular heists are as a whole, it's easy to see why the series of Triad missions in Las Venturas is so memorable, despite not actually being a part of the main story.
Note: This list is subjective and reflects only the writer's opinions.