6082-T6 is easy to get hold of, easy to machine but a little on the soft side. 7075-T6 is stronger, harder but is harder to machine. I'd go 7075 for the arm, 6082 for the head.

Avoid 1050 and 5021/5083, those are cheese grade, and 2014 is more for cold forming as its pretty malleable. There is also 'Eco' aluminium out there. Great for the environment... not so good for robots.