Check this website, it has the complete pokedex for all the Pokemon games (which includes the location for where and what levels the Pokemon can be found in the wild) as well as other helpful links like berry and item guides.
serebii.net
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There is no best Pokemon on Pokemon HeartGold.
there is no Pokemon Mansion in Heartgold.
They are not in Pokemon HeartGold
No, but you can trade Pokemon only in HeartGold to Black
Technically,yes you can get Arceus in Pokemon Heartgold,but only by trading it to your Pokemon Heartgold game from Pokemon diamond,pearl or platinum.