Formable as in casting, extruding, bending or other? Depends on your application. Formability can be gained with many allows, but not necessarily in their higher tempering ranges. For instance a 6061-T0 would be "formable", but using a 6061-T6 the "formability" would be reduced, depending on the forming required.
Depends on the bend radius. You might consider using a lower temper with ageing to gain strength. 60 series allows work pretty well for this. Bending at T4 temper then ageing to a higher temper is possible.
Alloy 1100 (essentially pure Al) in the annealed condition will have the highest formability, but has an extremely low yield strength (~ 35-50 MPa). Alloy 3003-O (annealed) is also very formable with low yield strength.
anything can be formed ... suggest you consult a sheet metal mechanic's handbook.
is heat treating out of the question ? (it is common to heat treat to O-condition, form (2024T3,7075T6, ...), and then heat treat back to the desired strength)
if it is then each alloy has it's own rules as to what can be achieved easily ... 2024T3 will happily bend at radii of 3t, 7075T6 likes a bit more (5t) but can work to 3t if care is taken ...
Lead is used in many of the alloys to increase machinability and possibly formability. So be careful if you are exporting to any countries that restrict lead on imports that need to meet RoHS compliance. (unless of course you claim exemption)
Agreeing with previous posters, 1100, 3003 and 5052-H32 are going to be the most formable and widely available. In extrusions, 6063-T52 is nice material to work with.
Agree with the others, 5052-H32 is very common and easily formable. You need to use a decent bend radius to avoid cracking, a radius of about the thickness is usually enough. Also forming a hem will cause cracking.
