I'm going to argue that odium is number one, not harmony.
harmony has the most 'raw power', but odium is far older, and has far more tricks up his sleeve. He has more experience in shard to shard combat, is capable of destroying shards (which ruin didn't seem to know how to do). Its hinted he may have been involved in whatever happened to aldonosium. In a fight between the two, my money is on odium.
Also, if we have four splintered shards, aren't their 11 living shard bearers? because harmony has two?