Aside from the 8 days (IIRC) it took them to find a set of car keys that were in plain sight -- in a room that had been searched multiple times before but which, once the Manitowoc County Sheriff's office shows up, miraculously appears -- the fairly obvious planting of the blood evidence is what cinched the deal for me. There are no finger prints in the car, but there are big blood smears? The former makes it sound like the car was wiped down (possible) while the latter makes it clear it wasn't. If it wasn't wiped down, there should have been finger prints, but there weren't. I suspect it was wiped down, and the blood was added ex post facto (as evidenced by the tampered with blood sample in the possession of the Manitowoc County Sheriff's office, which clearly had the seal broken and the vial penetrated with a needle).
Also, while I don't think Avery is a genius, I suspect that, in the several days after the murder and before the reporting of the victim as missing (and the police investigation), he would have gotten rid of the car. If he committed the murder, I'll bet he did get rid of the car. As pointed out, while he's not a genius, he has a car crusher on site and could easily have crushed the vehicle. Having those folks just wander right up to it and find it is suspicious, too, but that could just be dumb luck -- and I doubt Avery would have given permission for folks to search the yard if there was any possibility that the car was just sitting there on the side of the road waiting to be found.
I'm not sure what to make of the portions of the body that were found in the "burn pit." Maybe he put them there, maybe someone else did. I truly don't know. If there wasn't obvious planting of evidence elsewhere, I would have assumed that he did it and finding the body there would point to his guilt. However, all the other evidence makes me doubt the burn pit evidence, too.
I'm not at all sure if Avery is guilty. He seems like an asshole, and I don't like him, but I don't know that there's enough reliable evidence to convict, esp. given the circumstances with the Manitowoc County Sheriff's office.
I don't think Dassey is guilty. His confession is a joke and a miscarriage of justice. He's clearly not all there, and had wildly ineffective counsel. Every time his lawyer came on screen I wanted to punch him through my TV.
I don't know who is guilty. What I am pretty sure of is that crucial evidence appears to have been placed at the Avery residence by a third party.