The most effective exercise to build muscle is not really a workout. For me it is biking to beaches. I bike to a far away beach. Stopping is not an option because then I would be stuck in the middle of nowhere. I increased the distance or speed every time I went for a bike ride until I could bike 38 miles. In a few months I gained 30 pounds of muscle and that was biking one day a week.
To work the arms I would recommend kayaking or canoeing. You could run, bike or row using exercise machines but it is easier to force yourself to exceed your limits if you actually go from one place to another. For me the most effective way to build muscle is to travel under my own power from one place to another. The second best way is to use an exercise machine that tells me how far I would have went.
The most important thing is to improve every time or almost every time.