But seriously, the three legendaries in Pokemon Gold is probably the hardest to catch. I spent months trying to get them till my cousin that was jealous about my non-cheat lvl 100 Typhlosion and deleted my file. And hacked into it and broke the save option. He could've waited till I caught at least one, but no.
lugia made an apperance in saphire right after the movie came out, and i heard you can only catch if you the right variables ( amount/level pokemon or time or progress ) idk, he escaped me every time... i played for 18 hours straight trying to catch him, just saveing then loading and trying again, i met him twice....fail..
They r all easy just get an action replay and get 9999 master balls lol The hardest for me was girantina in a poke ball. It took about and our, but I did it
Chaining for shinies is to me the worst thing ever. Ever. It's just so tedious.
But if I have to pick a single Pokemon, I remember having an awful lot of trouble with Mewtwo back in the original games. (Hoo, was that a long time ago.)
I'm more of an original 151 kinda guy. After a while, the new pokemon weren't all that new... they're pretty much the same thing w/ a different name. I have a few humorous cartoons that I've found during my travels throughout the internets... they're memes, so I have no idea who the original person was who created them.
Since it took me over 10 years to figure/find out how to catch Mew in the Red/Blue/Yellow games I'm going to say that my vote goes for mew(who is actually really easy to catch...oddly enough)
in heartgold i've been trying to catch that latias (or os) for ages, but i agree that any pokemon that runs around is trouble, you have to make an effort to get in a battle with them, let alone catch them!
Usually the hard to catch Pokemons are those you have random encounters with and then flee. But by far for me Mew-Two was the hardest. Many of you think this is easier than the others but try doing it with 5 regular poke-balls and each pokemon close to fainting.
As someone else allrdy mentioned, I had a heck of a time trying to catch the 3 legendary's in the Crystal version, and another one that gave me a hard time was that green dragon thing in the Ruby/Sapphire Series, I believe it was.
Oh lord yes, Nicho. I'd forgotten how hellish getting a Feebas was back in Ruby. I heard it's even more difficult now, but I pretty much gave up on it.