Shichibukai strength is so damn hard to really pinpoint. The only way I can do that is try to find objective matchups they have in common, where both competitors are at a reasonable level of health and stamina:
#1) Blackbeard, Crocodile, and Moria: LUFFY -All have fought Luffy, and all have had varying degrees of success versus him. Although you can attribute Croc or Moria's losses to various things (not taking Luffy seriously, being too lazy, etc), the fact of the matter is that THEY LOST!
Blackbeard, on the other hand, DIDN'T! Although their fight was in a much different context, everybody walked away from that matchup thinking that Blackbeard was clearly superior to Luffy in strength.
Therefore, BB > Croc & Moria
Croc and Moria hmmm…Both brought Luffy to his limit, and both "beat" Luffy. However, the power-ups and effort Luffy had to use to beat him were much greater than the ones versus Croc.
Therefore, BB > Moria > Croc
Now, Jimbei fits really, really easily into the equation for me, since he has a perfect edge versus the bottom two, but still has expressed genuine concern over BB's strength. Therefore:
BB > Jimbei > Moria > Croc
The other four are difficult to place on the chart. Not surprisingly though, they're all active, so I have a hunch that every remaining Shichibukai is more powerful than both Moria and Croc, but other than that, we don't know.
Hazarding a guess, here's what I'd go with:
GUESS: BB > Doflamingo > Kuma > Mihawk > Hancock = Jimbei > Moria > Croc
Moria and Croc both appear to be getting a post-time skip power-up though, so anything can change. As for my high ranking of DD…I just have a hunch that there's a good reason why we know next to nothing about him. EVERY other character we have background info on, Devil Fruit info, etc...But not Doflamingo!