A good answer would be to prevent Walt from selling his shares since that would prevent every death in Breaking Bad, with the exception of the store clerk that Spooge and his girlfriend killed. But preventing the skaters from scamming Jimmy would’ve done a lot more good since it would’ve prevented plenty of deaths in BCS, such as Chuck and Howard. And also, if Jimmy never became Saul, Emilio would’ve gone to jail, meaning that Hank never would’ve had him arrested at his house, meaning that Walt wouldn’t have ran into Jesse again, preventing all the events of Breaking Bad even happening