1xxx, 3xxx, and 5xxx are non-heat treatable (harden able by applying higher temperatures). The way you harden these alloys is through imparting strain. Strain can be imparted through processing from the rolling mill, or through forming, bending, & other work hardening.

1XXX is the softest, 3XXX, next softest, and 5XXX is the "hardest", and strongest of the 3. If you by any of theses in the "O" temper, that will be the softest state you can buy that particular alloy. If you want strronger alloys, you will look at 5XXX series an look at "H" tempers. For example, 5052-O is a great alloy and fairly strong, with good formability. 5052-H32 on the other hand will be much stronger than 5052-O, but will also have less formability(ductility). There will always be a tradeoff...

2XXX, and 6XXX are the most common heat treatable (increase in Tensile and Yield, reduction in elongation when exposed to higher temperatures) alloys, and they are generally stronger in the as recieved (typically T4) state. However with the characterstic of these alloys, you don't get the rue benefit of its strength unless you apply controlled heat (350 F for 30 min). You will see the properties (TS and YS) increase by30-100% depending on the alloy. Pull it out an let it cool to ambient temp slowly on its own. Do not quench it.

All that being said, you would be better with 6061 than any 5xxx alloy if it is in the "O" temper. 6061 is a great general purpose alloy and if you really want to get the added strength, throw it in the oven after you do all your bending, drilling, machining, etc. and leave it in at 350-370 for 30 min.

7XXX is also heat treatable, but tends to be quite pricey. It will be a little tougher to work with. My recomendationwould be 6111, 6022, 6061, or 6016. Of the 6061 is the easiest to obtain.