Latios/Latias were pretty hard for me to capture because back then, I didnt know how to find it. I just kept walking randomly hoping to find it. Also my masterball mysteriously disappeared one day so I couldnt use that.
Also, why is everyone picking Mewtwo as the hardest legendary to catch? I just used my Charizard to weaken it to yellow and I caught it with a pokéball first try. It was the easiest legendary to catch.