Of course, that power ranking is only based on pure power. In other words, circumstances are not factored in. This is the reason why things like UBW!Shirou beating Gilgamesh is possible, and why (over all) he's [Shirou] not that powerful. UBW though is an extremely circumstantial ability. With it, he could theoretically copy weapons that were weaknesses against various people, and easily defeat them that way.
I should note though, Gilgamesh is still the King of Carelessness. As noted with circumstantial situations, it's possible for him to be defeated by someone of much inferior ability. I'd imagine that Ciel could probably off him in the same way that Sakura did, by catching him off guard with her 'regeneration' abilities and wounding him fatally.
Of course, this is only on the note that Gilgamesh would attract THAT much attention of getting someone else from the Church involved.
Something off subject to note is that I recall from somewhere that Shiki probably can't perceive Servants without frying his brain. Thus, while the fight of "Shirou VS. Shiki" would end with Shiki winning, he'd more than likely lose against say... Archer.