Viserys inherited the best realm he could have inherited. Decades worth of peace, the faith already put in line, the kingdom secured, infrastructure, already built, and plenty of dragons.

Literally, he only had one job. Ensure a peaceful succession and keep his family in line. He did neither.

Empowered his sociopathic violent second coming of Maegor younger brother. Put his unqualified brother in multiple different positions of power that he screwed up every single one to the point that the last position of power that he held created a faction of individuals that had more loyalty to his sociopathic, younger brother than it did him. 

Allowed the war in the step stones to even occur, and then after it started, did absolutely nothing and allowed said sociopathic brother to handle the situation, then abandon the situation which led to trouble further in his rule. 

Took a second wife after his first died, who gave him three healthy sons, but instead of naming Aegon as his undisputed heir, which no one would have pushed back against not even the 8 year-old Rhaenyra in the book and the teenage version of the show would just have to stomach it like every other reasonable person would have to in a situation where she had no power but he didn't do that! Going against every tradition, law, and custom in the entire realm. Essentially pulling a Maegor by putting the king's word above all else even if it is a foolish, prideful, disastrous one. 

Also, I love how team feminist doesn't even comment about a man using a woman's body, forcing her to have multiple different children and then just abandoning the children because he doesn't wanna play with that wife anymore

The most insane thing is when it is offered for Aegon and Rhaenyra to marry to settle the succession, which even makes more sense in the book when their age gap is even smaller, he denies it. The one decision that would've prevented any of the factions from being created and pleased almost everyone with the compromise he denies it.

Raised an entitled, spoiled daughter, who willfully created not one, not two but three different bastards with the same man. Instead of disinherit Ing, her immediately he instead stood by her allowing further mockery to not only happen to his own house but the house of his greatest allies house Velaryon, and then cut out the tongues of five different individuals from that house when they took issue with bastards inheriting over them.

Then when his daughter married the very man that he changed the succession because of, he doesn't disinherited her and she has two trueborn children with him. So not only has his sociopathic brother that he has exiled multiple different times now have a path back to the throne, but no matter what, even if there isn't a succession crisis between his oldest son and oldest daughter, his daughter has insured that there will be a succession crisis between her bastard sons and her trueborn sons sooner or later.

And either if it is because of ignorance, denial, or delusion, instead of trying to find a way to appease the growing faction of the greens or pacify  
the increasingly hostile blacks by finding a way to mitigate peace between them, likely marrying their children off to each other: Aegon to Baela, Aemond to Rhaena, Helaena to Jace, and punishing Luke for mutilating Aemond, or doing anything of the sorts, he does nothing except make the black fact wear green and the green faction wear black.

And even better allowed his heir, his successor, to completely abandoned Kings Landing for almost a decade and be removed from the public eye.

On top of all that the only time his rule actually became somewhat stable was when Otto and Alicent we're doing his job for him.

Viserys get too much slack and merely called indecisive when in truth, he made every wrong decision he could from beginning to the end of his life. Never making one smart decision and hindering everyone else trying to solve the problem.

He insulted and disgraced House Hightower, House Velaryon, and didn't just allow, he personally made his own house, weak, divided, and hostile due to his own actions. There is no worse king than him, especially given the incredibly prosperous realm he inherited.

Viserys is the worst King Westeros ever saw and is the true destroyer of house Targaryen