Hi.

I'm getting close to buying Karthus. So I'm wondering, which runes would be very good. Some suggest hp and armor, some ap and mana. I myself am considering cooldown reduction and magic penetration. Does anyone have really good suggestions?

probably the best is
red - magic pen
yellow - HP
blue - AP or cd redux
quint - hp

^What he said. It's open to a lot of leeway however. What is not really desirable is AP or mana imo. Karthas doesn't have to worry about regen really (at least as far as runes) and AP doesn't have a huge effect on him. HP is always good (and the most effective rune, btway) and magic pen and cooldown reduction are always good on him.