In short terms, Lugia or Ho-oh.
The best Pokemon in Heart Gold depends on how you use that Pokemon. For example, a level 1 Rattata holding a Focus Sash can beat a level 100 Lugia with Endeavor and Quick Attack. (The Lugia must attack in order for this to work.)
IVs or Individual Values are another way making a Pokemon strong. However, to get all perfect IVs is nearly impossible. They are also very hard to manipulate.
Moves, Ability and Base Stats make a Pokemon strong. Pokemon such as Jumpluff can use Sunny Day and use it's ability "Chlorophyll" (Boosts Speed 1.5x in sunlight) and with it's 115 base speed, it could outrun almost any Pokemon.
Well it depends on what you think but it would probably have to be a legendary Pokemon. I would say ho-oh is the best.
shiny rayquaza with wondergurad
The best place to evolve a Pokemon in Pokemon Heart Gold is at Mt. Silver.
grass
Aura Beam is in Pokemon HeartGold.
That is impossible in Pokemon HeartGold and SoulSilver.
There is no best Pokemon on Pokemon HeartGold.
there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
my mum she is da best Pokemon Eva
shiny rayquaza with wondergurad
The best place to evolve a Pokemon in Pokemon Heart Gold is at Mt. Silver.
soulsilver and heartgold, it has the most trainers
probably the legendaries"in my opinion btw"
Use items.
there is no Pokemon Mansion in Heartgold.
They are not in Pokemon HeartGold
Level up your best Pokemon to at least level 60.
Magicarp is probably the worst but evolves into one of the best Pokemon in the game.