I Agree, to me all of them are simply people who had their strengths and weakness but in the end one person can only simply do so much and it seems much of an emperors reputation was outside of their Control. Some of them like Augustus was probably very lucky (or unlucky?) to be alive just at the right moment while others was pretty much doomed. In the end it was other people who wrote about the emperors and some of them was made into good ones and some was made into bad ones which to me seems to be tied into politics, often murdered emperors was made into bad ones even if they was no worse than their predecessors or successors while others was made into good ones even if they are simply a single person whose success is built on the work of other persons that are maybe no longer remembered.
Only way I could really give a ranking would be if I had a time machine or something like that and could exactly see Everything that happened at the time and after that make my judgement..
let's take the emperor valerian for example, he's at -9 for the moment, he inherited massive internal problems (which he was partly at fault of, in a way he was the number 2 of gallus when he was killed by his legionaires in favour of aemilianus, so 2 choices, surrender to new emperor and be executed and watch another wana-be emperor rise from his ranks or press on to become emperor himself), revolts in gaul, a gothic invasion and a persian invasion
so he send his son west to deal with the revolts in gaul and he himself went east to fend off the persians, on his way there it's said that he ran into the goths, apparantly there are no proper sources but the goths weren't seen in the empire for another 20-some years
then his army got the plague and then the persians arrived, surrounded him and his remaining legionaires and he was captured
looking at all this, how is any of this his own fault?
yes, his realm saw some of the worst catastrophies of the roman world which makes it a candidate for -9 for the times he lived in but I doubt it was because of his lack of abilities
you might as well say: "I take points of titus because of the vesuvius errupting"