@Ivotas:
While I like the ranking you´ve done there SaiyaJedi I have to this ranking of Stephen´s site is definitely correct. You see it is not a ranking that Stephen made up himself, but it is one that Oda himself established in one of his SBS. So this is definitely the ranking with which we should go. 
[edit to remove irate ad-hominem; rest of post remains.]
Oda wrote the rank chart in Japanese. Both stephen's and my listings are based on Oda's own chart; they're just different translations. Japanese and English don't have a 1:1 correspondence, and it just so happens that the language uses the same words to describe military ranks across different branches of armed forces, that are called very different things in English.
The problem with the two "captain" ranks in Stephen's own chart, is that taisa only means "captain" in the sense of a Navy (which is what the Marines in One Piece are), where in other branches, it's the equivalent of "colonel" (such as the colonels in the Red Ribbon Army of Dragon Ball). Tai'i only means "captain" in the sense of the Army, Air Force, and true Marines (that is, mobile assault troops). In Naval rank, "tai'i means "lieutenant."
Generally speaking, I like to stick with Naval rank whenever possible, the military in the series is called a Navy (Kaigun) in Japanese. It gets more confusing in lower ranks, where the modern Japanese Self-Defense Force uses different ranks between the naval and other branches, which is why I included both Navy and Army equivalents of the lower ranks.
By the way, this discussion isn't unique to One Piece. It's been a major point of heated discussion in the English-speaking Gundam fandom for well over a decade.
A useful link:
If you spoiler me, I will eat you with tater tots.