1. The One Magic - ? - It must/should be a combination of all types of magic, pure magic, in any of its manifestations.
2. Dragon Slaying Magic - Acnologia - When at its peak, this magic is pretty godly, and when you turn into a dragon, you pretty much can do anything.
3. Etherion - Magic council - Can pretty much nuke a country
4. Age Seal - Dimaria - She can probably stop time up until her MP runs out, which must be a looong time.
5. Mass manipulation - Brandish - In the event it has no limitations and can shrink anything with matter, you can even shrink MBP, which would render Bradman's ability ineffective - just a supposition. With absurde amounts of MP you could shrink entire countries if you wanted.
6. Fairy Heart - Mavis - While it is infinite MP, it is not infinite DC. I mean you can keep throwing water buckets to the sun infinitely, it won't put it out.
7. Bradman (Three seals + 9D Curses) - Macro alone is haxy enough...
8. Transport - Jacob - The ability to transport anything to another dimension- which you control (and can kill anyone in it, anytime) is pretty haxy.
9. Hybrid theory - God Serena - Posessing 8 DS types of magics is quite impressive; although it you don't excel at any... well... not that much.
10. Historia of corpses - Neinhart - Interesting magic, but you can only summon people below your level...
11. Heavenly body magic / Celestial magic - Jellal, Lucy - Anything that involves "heaven", "universe", "stars" must be powerful xDD
12. Vampire magic/Moisture sucking - Hyberion, Ajeel - Seemingly, they allow you to 'extract' life out of the body.
13. Stealth - Jacob - Imagine. You can't know where your enemy comes from, he has slashed your throat before you even notice.
Honorable mentions
Contradictory Curse - Mavis, Zeref - While it is powerful (in the sense it can kill anybody), it is uncontrollable.
World Reconstruction Magic - Irene - Effects yet to be shown
God Slaying Magic - Shelia, Orga, Zancrow
Devil Slaying Magic - Gray
Crash - Glidartz
Jutsu Shiki - Freed
Fairy Spells (Fairy law, glitter) - Mavis, Makarov, Laxus, Cana
Light Magic - Makarov
Territory - Minerva
Memory Make - Rufus
Reflector (Illusions) - Macbeth