any of the three cat-things m( i forget their names, fire,lightning water,) lugia, mewtwo,and the three legendary birds ( it was a b**** even getting to them...)
Hard to catch: well disincluding the catch rate statistics, i think that the hardest to catch would definately be the Lugia in XD Gale of Darkness, you don't get any special balls, only ultra balls, and you can use master ball but still, without that. Another really hard to catch Pokèmon that is annoying to find in wild is Zangoose in Diamond/Pearl/Platinum.. Don't ask why.