The one you currently have in custody is the easiest to convict. Especially in higher profile cases where there's political and social pressure.

Are you entirely sure? Someone's race, ability to pay a lawyer, and prior history can be huge factors in convicting them.

Convicting a poor black man who dropped a cigarette butt at the scene hours prior to the crime may ultimately be far easier than finding the actual perp.

Naw the easiest person to convict is some random stupid kid who you can pressure into admitting to any crime you want.

One of the big problems is the anti scientific method. the scientific method consists of formulating a hypothesis and a method of testing the probability of failing to reject a null version. in common speech this means make an assumption, observe data/evidence and alter the assumption to fit the evidence. in the case of anti science such as forensics or investigations of crime, the opposite occurs. the assumption or theory is made the data or evidence is observed, and discounted or re interpreted to fit the theory.