I am told that the name "Mikko" comes from Archangel Michael, one of the holiest of them all. He was the right hand of God, slayer of the Dragon, and hero of the Final Battle. Other popular variants of the original are "Mika", "Miika" and "Mikael".
One of the most famous Mikkos would probably be Mikko Kivinen (an actor, formerly a singer in the group "Lapinlahden Linnut").
I was given this name 21 years ago, and I'm still not bored with it. It has served me well, although some inconsiderate people have tried to make some "cute" nicknames based on it. Most infamous example would be "Miksu", which I hate.