Not a trailer application, but on the drop outs I've done myself on some of my hack and weld jobs I've used 3/16" to 1/4" plate steel, either soft easy weld stock or stainless. Going thicker on the soft stuff and going down to the thinner 3/16 on the stainless which is harder stuff. YMMV but that is what I've done but I tend to overbuild when it doubt.

On a side note angle iron wouldn't be my first choice. I prefer tubing specifically thin wall (0.060"-0.065") square tubing mainly because its easier to cut and weld and get things straight then round tubing.