What pokemon is the hardest to catch?

Beldum and its evolutions, since they are nonlegendaries with a catchrate of a legendary.

Well, Azelf in W2 was the only legendary, which gave me trouble, catching. All other legendaries needed just a couple of balls, but this one took me multiple trys (save reloads).

any roaming pokemon generally...well maybe except thundurus/tornadus it was pretty easy to trace AND catch for a roaming pokemon but things like raikou suicune and entei are hard to catch as hell.