I always thought that the ranking for the strength of the blues was determined by the average bounty (in this case, bounty is a measure of strength) of their criminals.
We however can't determine where the North Blue, or any of the other blues besides East blue, rank in terms of strength. Whilst we may know a few remarkable individuals from the other seas, they do not represent the majority; they may be the Luffys and Zoros to their own respective sea: the extreme end of the bell curve. The only thing we know for certain is that the East Blue is the weakest, with an average bounty of 3 million beri.
[hide]From Stephens scripts: One Piece, Volume 6, Chapter 51
Mihawk: I am not a stupid beast who needs to use all of his power just to hunt rabbits.
You may fancy yourself a renowned bladesman,
But of the four seas divided by the Red Line and Grand Line,
East Blue is the WEAKEST of them all.[/hide]
[hide] From Stephen's scripts: One Piece, Volume 11, Chapter 96
[World Government Forces, "Marine Headquarters"]
Officers: So what you are saying is that this group
can no longer be contained by our branch forces…?
Speaker: "Buggy the Clown," 15 million berries.
Pirate Fleet Admiral "Don Krieg," 17 million berries.
Fishman Pirate Crew "Arlong the Saw," 20 million berries!!
[Marine Headquarters Lieutenant Commander]
Brandnew: These pirate crews have breached the 10 million berry mark,
high above the East Blue average of three million,
but they have all been neutralized.
30 million might seem like an extraordinary amount for an initial bounty,
but we feel that it is entirely appropriate, given the circumstances.
We must pluck out this seed of evil now to prevent the possibility of further damage!!!