Lina Inverse has the Giga Slave, a spell that calls on the Lord of Nightmares, the god of all existance. If she loses control of the spell, she'll destroy all existance. I'd say that's pretty much up there in High Tier at least. Even without the Giga Slave, her Dragon Slave spell is the same as a nuclear bomb.
Motoko Kusanagi litteraly becomes a god in the GITS manga, but her hacking, leadership, and tactical skills easily cement her as no less than high tier.
Spike Spiegal... There was nothing in the universe that could actually kill him except Vicious. He was fated to not die until he took out Vicious as well.>>11611408
Best one so far. FUCK YEAH! LUFFY!