there is no best spot
arceus
a lot of legendary Pokemon
That would have to be Dialga.
a master balls can unfailingly catch and Pokemon unless the Pokemon owned by another trainer and it is best not to waste them on weak Pokemon easy to catch and use them on dialga and other legendary Pokemon you only have one chance to catch in the game.
Best non-legendary water pokemon would be Gyarados. Best legendary water pokemon is Kyogre.
Palkia
You can catch more legendary pokemons on platinum. So you can catch more on Platinum, then the other games :)
That's a matter of opinion(myne is Giratina)
The safari area
grovile
in pearl palkia in diamond diagla
try serebii.net its got a lotta Pokemon stuff