San Andreas is still the king.
Let's face it, people don't play these games for the storyline.
That's the problem both IV and V have. The games can't make their minds up if they want to be a realistic crime story or a comical video game. IV lost me with it's storyline completely, and V's was much more funny and had it's moments, but it too had the same problem from going to being funny one minute to trying to be emotionally invested in the characters.
What made SA, LCS, VCS, VC, and III what they were was the games didn't take themselves seriously with a bad melo-drama. The characters themselves were meant to be like jokes that you could not take seriously, and that's what made them memorable.
The biggest issue I had with IV was it just didn't feel fun. I play these games to have fun. I don't give a flying f*ck about the characters and backing story. There was nothing to do in IV. Nothing fun, outside of a few memorable missions, and even these were tedious. For a game that is all about stealing cars, they made the driving controls absolutely terrible, and a pain in the ass just to drive around. For a game that is meant to make a joke out of crime, it takes itself way too seriously.
Thankfully V fixed most things that were wrong with IV. The driving is a hundred times easier again, the shooting mechanics are better, the game has limitless FUN activities to do besides the 'hang out' and dating crap that plagued IV, and most of all this is far more fun than anything that was in IV. Yes the storyline wasn't all that great, but people act like IV had this amazing story when it was nothing but a laughable Scorcese rip off. It was so unbelievable while it tried to take itself seriously. San Andreas to me, had the best storyline. It tried to be serious in times, but it was all in all goofy and not meant to be taken seriously. It also had a great antagonist that felt like an actual threat to the player where the antagonists in IV and V are predictable and feel like card board cut out bad guys.
San Andreas was the high point of the series and nothing will ever top it again.