Summary - T. Shiki and the 27 have been removed from the rankings due to some debate
What we definitely know:
Aoko is no match for a serious Caster.
Divine Words are far faster than spells cast through High-Speed Incantations.
Avenger can beat any human being. Well, in the end, any evil caused by a person turns into an evil he committed, so that probably plays some role in it.
Rin is a better Magus then Kotomine, but he'll still win based on years of experience and physical skills.
Most magi might not seem to be that tough, but the fact is physical + reinforcement spells have given them superhuman speeds; with the obvious limits of a human being.
Gorgon looks a hell of a lot more powerful than Rider.
Iskandar had a Noble Phantasm that could match up with Gilgamesh.
A normal person will be torn to shreds by a ha Beast. One with only 10 years of martial arts training will just last an extra few seconds longer.
Dragon Tooth Warriors are pretty strong. Stronger than Kirie and Fuji-nee at least.
A Servant with an average ranking (B) Noble Phantasm that can be converted into numbers (like a Broken Phantasm, the thrown Gae Bolg, Invisible Air) can fight against all of the Dead Apostle Ancestors, Kouma Kishima, and Aoko. However, those Noble Phantasms with special effects such as destiny interference (thrusting Gae Bolg) are somewhat different and can't or shouldn't be used as a rule of thumb, even if they're lower than B.