Since there is so much evidence about what a scumbag Renly was why on earth would the show writers make him the best of the three Baratheon brothers? It makes no sense to me.

Because GRRM, in his tricksy wisdom, also made Renly intelligent, funny, and genuinely charming and then made his main opponents people who aren’t. 

When we first encounter Renly in Sansa I and Eddard III, not only is he a young Robert without the alcoholism and Targ-murder-boner, but he’s also making fun of the Lannisters who we’re being primed to hate even more because they’re trying to execute dogs. (Although if you think about it, isn’t it interesting that Renly comes off so well despite not lifting a finger to actually help?)

Then when we encounter him again in ACOK, he’s being contrasted against Stannis, who we’re also primed to dislike (so that the face turn works in ASOS). Remember, Stannis is introduced allowing walking empathy magnet Maester Cressen to be humiliated, and the next image we have of him before he meets with Renly is him joining a scary cult (another example of how priming wrong-foots people: Melisandre). And again, look at their meeting:

Again, on the surface, Renly’s the one with the better japes, the peach, and he’s the one who’s going to get horribly murdered so there’s the sympathy factor as well. 

The case for Renly falls apart when you step back, ignore all of the surface qualities, and ask yourself what has he actually done and what has he actually said. And that’s when you start to see all of the subtle thematic and character work GRRM has been doing in the background. 

My guess? Benioff and Weiss aren’t very good at literary analysis and simply missed that second layer.