Ultimate tensile strength
Lets start by making the usefulness of an alloy measurable. I'll use the unit MPa (ultimate tgensile strength).
For the sake of comparability here some values:
* human skin: 20
* aluminium: 45
* gold: 100
* copper: 210
* iron: 350
* brass: 550
Judging by this table brass appears to be the best choice for armor and indeed it is. At leas as long as you don't want to move so much. Brass has a higher density than iron (8.7 to 7.8). Also you need to keep in mind that brass is more expensive than iron.
Pricipation hardening
Now, in order to get some decent alloys your civilization should needs to know the art of prcipation hardening which ain't that hard if you use materials which have a low melting point like aluminium. Take a look at wikipedia for more info.
Magnesium Alloy
If your civilization discovers magnesium (high probablility as it is earths fourth most abundant resource) you have another high potential component for creating powerful alloys.
If you'd put aluminium and magnesium together you can create a very powerful alloy with a MPa of 450 (Source: German Wikipedia, search for Aluminiumlegierung. Sorry, can't post more than one link) and very low density, making it lightweight armor more powerful then iron. Using a lightweight material like this would allow you to construct plate-armors in which you could move around more freely or a chainmail nearly as light as a leather armor but much more piercing protective. Using it as a weapon on the other hand has the weight disadvantage as you need some weight to cut through armor or break some bones. 561
You should therfore not use it for a non-cutting weapon.
There are much more aluminium - alloys and magnesium alloys. Take a look yourself, but most of these alloys demand a higher level of technology. Take a look at the aluminium-zinc alloy with an MPa of 700 for example.
Cermaic Armour Plates
You could use ceramicplates in vests to get a high penetration-protective armor. Ceramic is used in a lot of moder composite armors and bullet proof vests. Ceramic armor is however a one-use armor as the stone tends to shatter after receiving a blow or two. As ceramic is much lighter than steel an iron, ceramic vest seems to be useful to skirmishers - fast attack, fast withdraw, minimal casualties.
Silk Armour
This sounds ridiculous at first, but the mongol horde used silk armors which were much more penetration-protective than european plate armor. Above the thick silk armour they've worn a scale armour to protect them from cutting damge. The silk armour was super effective as it was very light an hardly penetratable by spears and arrows alike. Silkworm silk has a density of about 1.3 and an MPa rating of 500. It's obviously not suitable as a weapon. 523