Well, for me, my easiest legendary pokemon catch (w/o master balls, duh!) was the lvl70 ho-oh on my silver version, believe it or not! It took me 2 maybe 3 turns to weaken it, when all it would use was sunny day, and then i caught it in the first ultra ball i threw. The odds must be phenomenal! Sadly, this was like my 4-5th try (i saved before battling it) overall, but still! I'm just lucky sometimes, i guess...