Here is my personal ranking. (note: I believe 1-4 are almost to close to call)
1)Donflamingo. He was controlling some of the highest ranking people in the world government that has to be a glimpse at how powerful he really is.
2) Mihawk is on par with Shanks that has to be saying something
3) jimbei just the fact that they havent shown him makes me think he is going to be very powerful
4)Kuma if he ever fought to his fullest would probably be one deadly s.o.b
5) Blackbeard. he is strong with a good ability but i think if ace was given a rematch he could take him
6)boa hancock. she is the bane of most men but seems easy to overcome with dicipline.
7) moria without thriller bark i think he is not very effective but who knows maybe he has different tactics when he fights for himself.