Different story missions affect the GTA 5 stock market in different ways.
First, you should wait as long as you can before you do the Hotel Assassination mission, which is required to continue the story. All of the other assassination missions should be saved until the end.
Second, use the following information about how different stocks react to story events to invest your money wisely.
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LifeInvader and Richards Majestic
You should not invest in LifeInvader or Richards Majestic. LifeInvader will decrease in value after Friend Request, and Richards Majestic decreases at a later point in the game. The key issue with these two is that they never regain their original values.
Vangelico will decrease in value after The Jewel Store Job and increase after Minor Turbulence, for about a 40% return if you buy at its low and sell at its high.
FlyUS will decrease in value by about 50% after Legal Trouble and increase to its original value after Meltdown, for a 100% return if you buy at its low and sell at its high.
Cluckin’ Bell will decrease in value after Paleto Score and Gruppe Sachs will decrease in value after The Big Score (Subtle Version). These two, along with Augury Insurance and Merryweather (which do not decrease in value during the game), will increase for high returns based on your ending decision.
The remainder of this guide contains spoilers for the ending. Do not continue reading if you don’t know about GTA V’s ending choice.
Cluckin’ Bell will increase to its original value if you go with The Third Way (Deathwish) ending choice, for about a 65% return if you buy at its low and sell at its high. Gruppe Sachs will also increase to its original value, for about a 30% - 35% return if you buy at its low and sell at its high. Augury Insurance will double in value, for a 100% return.
However, if you pick The Third Way, Merryweather will decrease in value by over 50% and never return to its original value. To make money by investing in Merryweather, you should choose The Time’s Come (Kill Michael). It will increase in value for about a 250% return.
These are all of the stocks currently known to be affected by story events.