Best torch angle for welding

I think you would be more likely to get a correct answer to that question on a "welding" forum rather than on a robot related forum.

That's a corner weld, more or less, so you can go with a 45 degree angle of the wire between the surfaces. You need to push rather than pull the wire forward, so take a sharp angle, a few degrees, in the direction of the weld.
Granted, I've never programmed a robot for Al welding, so take my general, not-actually-a-welder, advice with a grain of salt.

We don't weld AL but we use 45 deg on all our welds, unless space is limited. I agree with ClaudiuA about the push method so you don't create burn throughs. Good luck!

Your work angle (assuming 45°) looks pretty good as your weld looks evenly distributed between both parts. Try a greater travel angle (10-15°) push, which should turn out to be a cleaner weld like the end.