1. Song of Resurgence
This is undoubtedly the best overworld mugic there is. There isn't a single match that you will play in where this card will not be of some use. Just think about it, who wouldn't want 20 free energy?
2. OverWorld Aria
10 energy, and protection from those nasty underworld fire attacks? That sounds like a pretty nice deal to me! Even having a creature like Maxxor with a normal healing ability would still be able to cast this with some benefit. Just remember to use it early, but not too early. I have noticed some players using this mugic when my first attack is a fire attack, which is the wrong play. Even though Overworld Aria's ability would stop 5 points of damage that a fire attack would deal, you wouldn't benefit from the 10 points of healing. In the end, you wouldn't get all that you could get out of the mugic.
3. Refrain of Denial/Prelude of Protection
I had a tough time picking a third mugic for the Overworlders. Not because all of their mugic is so good, but because it is how do you balance their abilities with their costs. With this Refrain of Denial, you are able to counter any mugic, but you have to pay two mugic counters, which could be spent healing your creatures, or casting a more helpful mugic. Then there is Prelude of Protection, which is great, and may help you more than even Song of Resurgence, but it just isn't as versatile as the other mugics. You HAVE to cast this mugic before combat begins to receive any effect which means you will have to know before the battle that you will need the extra edge in battle, which would usually mean trouble anyway.