In a world where humans are physically stronger (say, they lift about 40-45k lbs), and probably gravity is 0.6 from Earth's, what is the best metal to make an armor from?
I'm pretty sure depleted uranium plates makes for the best armor. At least the US military thinks so because it is used in their state-of-the-art battle tanks.
If worried about (hypothetical) radiation dangers, there are tungsten alloys that are as strong as depleted uranium, but its really expensive to make.
I'd not use metal at all. I'd use human muscle. Whatever you did to physics to permit a human to lift that much mass would make it perfect for use as body armor!
Of course, the thing to remember about armor is it is not made to suit the individual wearing it. It's made to suit the individual wearing it and the particular attacks that one is trying to deflect. Without understanding all of the weapons these superhumans may face, it's never clear what the best armor looks like. For a concrete example, consider the difference between a kevlar bodyarmor vest and a full EOD Advanced Bomb Suit. Their materials are similar (both rely heavily on Kevlar), but their design is vastly differently. The ABS, for example, is carefully constructed to prevent overpressure from an exploding IED from rupturing the technician's lungs. The typical kevlar body armor doesn't have this because it's very bulky and uncomfortable and most individuals wearing body armor aren't facing enough IEDs to warrant the added discomfort and weight.
If you design an armor to maximize its weight you are aiming for killing its bearer, who will struggle wearing this heavy piece of sh...ield on the battlefield. Imagine a vest made of 10 cm thick reinforced concrete: max weight, max protection, null mobility.
Your design should instead aim at maximizing the energy absorption (you want the bullet or the blade to dissipate its energy while travelling through the armor, not the body underneath) while keeping it as light as possible, to let the soldier as agile as possible.
The densest metal is Osmium. But heavy is NOT the same as strong/resistant!
Osmium the densest naturally occurring element, with a density of 22.59 g/cm^3. Note that this is more dense than Uranium (~19 g/cm^3), depleted or otherwise!