Catching Heatran in Pearl. And Giratina, actually. Both were crazy hard for me. Hell, throw the legendary birds in there too.
One time I beat a good chunk of Lance's team with just a Wobbuffet.
Most recently I caught a Kangaskhan in the Kanto Safari Zone. Not outstandingly impressive but you don't know how much time I've spent looking for a Chansey, Tauros, or Kangaskhan. Too much!!