I'm gonna start this general list and lets all add our comments and evidence as to why that character should be placed there, The order is based loosely off power level.
I tried grouping parts of the list with "- -" to show that the order can be debated
Strong Tier is basically all the strong characters that are under Straw Hat Crew BIG 3.
Top Tier is basically all legendary characters who are known to be the strongest.
Gol D. Rogers
Golden Lion Shiki
Rival Tier is a mixture of future enemies as well as power levels within the range of the Worst Generation.
T. D. Law
Strong Tier is any character that is below the Worst Generation.