6061 as stated before "is the best choice for avaiblity, cost, machinabilty, and corrosion restiance"
In order to get the best machinablity out of 6061 you can try to heat treat the aluminum. Why heat you may ask?
1.) heat treatment changes the hardness property of
metal.
Why is hardness important to the machinability of 6061?
2.) Analyzing the stress vs. strain curve for
aluminum, as the material becomes harder, ductilty
will become less, therfore causing a reduction in
the ammount "toughness" of the material.
How is toughness important to machinablity?
3.) Toughness described analitically is the area under
the stress strain curve, in practical sense it
is the ammount of energy the material absorbs for
the formation of chips.
4.) Therefore the the harder the material becomes, the
less tough and better the machinabilty will become.
There are many different types of heat treatment, and quenching process available; also there are many different types of machine variables which will effect the machinablity of the Al 6061.
Do not give up on your search for the best material for maching, find info on the internet, and if possible a local univeristy might also be a good source of information.