there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
I put a link to a guide all about legendary pokemon in HeartGold in the related links.
In HeartGold you can catch Kyogre.
Articuno
mew
There is no best Pokemon on Pokemon HeartGold.
I don't think you can refight any legendary Pokemon except the wandering dogs.
migrate and trade
Kyogre is only available for Pokemon: Sapphire on the GBA. Ho-Oh is the legendary pokemon available to you in HeartGold.
No legendary can lay an egg except for manaphy which gives an egg to phione and you can not get manaphy or phione in heartgold
yes at supercheats.com/pokemon heartgold action replay codes
Ho-ohLugiaCelebiJirachiLatiasLatiosSuicuneRaikouEnteiZapdosMoltresArticunoMewtwoKyogre-HeartgoldGroudon-SoulsilverRayquazaPalkiaDialgaGiratina
Moltres is the only legendary Pokemon in Mt. Silver for HeartGold / SoulSilver only.