So perfect is impossible because it is subjective. For me this whole topic is about speed and redundancy where I cannot be defeated by any individual gadget whether phone/tablet, access to mains. So I am rarely with just one portable charger because what it that portable charger fails?
I find I disagree it seems with many owners in forums particularly Anker fans because I use USB batteries in a defensive way, i.e. if I stationary and not next to mains power I plug in my battery to keep my gadget charged - why? Because it is the most efficient and pragmatically best way to defend yourself from gadget power drain.
To "use a battery to charge a battery" it electronically quite dumb. Let's walk through the two scenarios:
1) Use an external USB battery pack to power a gadget which is 100% charged. The power goes out of the 3.7V battery inside the USB battery pack, is up-stepped to 5V, then through the cable to the USB socket of your gadget (say phone) and then it depends on if the gadget is charged or not. If its not charged then power is first used directed to the gadget's non-battery needs like cpu, screen, etc and then any non-used excess input power goes to the gadget's battery. If the gadget's battery is 100% charged then all the power through the cable is going into the electronics.
2) Use an external USB battery pack to power a gadget is not 100% charged. The power which goes into the gadget's internal battery to recharge must burn energy to get into the battery, and when it is then later used burns energy to get out.
So 1) vs 2) the energy goes through more pathways, more energy is lost.
So if you are carrying an external USB battery pack, you are going to get more total useful energy inside your gadget if you are proactively defensively keeping your gadget fully powered.
What that means is you're not in a rush, you are NOT having a gadget nearly flat and wanting to recharge it rapidly, that means you are NOT that worried about Quickcharge etc to make charging speeds faster. You are more interested in the weight of the external USB battery pack and its costs.
So in this defensive approach you do the weight vs mah and volume vs mah calculations. If you do this for all of Anker's batteries you see a few float to top. A really good top one is the Powercore 10000. The Powercore+ devices actually are often worse because often making the electronics larger with the likes of Quickcharge.
So then its a matter of just-big-enough battery for the likely worst case. This then leads into are you with any kind of bag.
If I'm not with a bag it is just clothing pockets. The Powercore Slim 5000mah is a decent density and is pocket friendly shape, more than the tube shapes like the Powercore Mini.
If I'm with a bag the bag is far more tolerant of size/weight. For this I'm usually carrying the Powercore 10000 and the Powercore Slim - both is to allow either one of them to fail, or for their total aggregate demand.
Two batteries can be charged concurrently for more rapid charging. That was true of all battery options until the new Powercore 26800 with dual input. It is actually not quite as dense as multiple Powercore 10000, i.e. you're slightly better off having 3 Powercore 1000 than 1 Powercore 26800 because you can charge a total of 6A input not 4A and have the same amount of output, but the probability of 3 shared-nothing independent batteries failing is statistically far lower than 1 larger battery failing.