Cresselia is not found in the wild in HG/SS, you have to trade it from D/P/Pt.
It's Darkrai. You can't get him in Heart Gold.
The new Pokémon games will be called HeartGold and SoulSilver.
There is no best Pokemon. Whether your Pokemon wins a battle depends on stats and skills. Every Pokemon is weak to something except a small select few. Even Mewtwo is weak to dark attacks. Suicune is...
breed it and it will come out with the same moves, apart from that no, not without cheating.