Awhile back in a MP thread, someone complained to Stormbinder (?) about a very tough SC that they were unable to counter. The only stat mentioned for this SC was that it was an airqueen and had Prt of 35. As I have looked through the equipment lists including the artifacts, I began to conclude that even without wishes, you could make some god-awful, one-of-a-kind SC's that would be hard to combat.
Consider a helio with crown of Amon Hotep, Bone Armor, Fire Shield, Sunslayer, boots of anteaus, etc. Not invulnerable, but very high MR and very effective in combat.
While could debate the merits of a single unit with that equipment (too many eggs in one basket perhaps), I am still curious what combinations of scripted spells and specialty equipment people have come up with that go beyond some of the run-of-the-mill SC designs. The design does not have to be repeatable, although ideally, if you mention it, you should have actually tried it once
One Last note: ignore 'wish' potential.