you can't.
you can catch a shymin in platinum with oaks letter which you can get with an event.
Shymin is an event Pokemon that can only come when the Pokemon Company wants to give it to the public.
by event.
shymin
the strongest Pokemon is arceus.then shymin,dialga,palkia and giratina.
the flower paradise on route 224
I don't think its there unless you have a cheat code
latias latios darkria shymin etc.
Nothing there is no action replay code to turn Shymin into it's sky form. You just give Shymin an item that makes him transform or go to route 224 I think
It is only found in that form in the new Platinum.
The shymin is only avalible in the wi-fi downloadable mission and the shymin in the mission jumps all over the place so its kind of hard to find it.
you have to give it a gracidea from the flower shop in goldenrod city.
There is no best Pokemon on Pokemon HeartGold.