What ball is best to catch beldum?

Well the masterball is always a good choice ;) Iwould go with a ball that fits the conditions. If its night you might want to chose a dusk ball, if its in heartgold try to find a Capricorn that makes a pokeball that would be good against a beldum(I don't know what ball each Capricorn makes!), a quick ball might be good if its right in the start of the battle, a timer ball if the battle has been wearing on for a while, OR you could weaken the Pokemon to a very low health point( if you got a scyther use false swipe to ensure 1 HP left) then use a ultra ball or great ball or even a pokeball.

I myself just recomend the last one because its pretty easy to do.

also if you have a poke walker, you could just have gotten winners path and catch it then.