Don't get me wrong, Hancock, Croc, Doffy and Jinbe - they all are fodders to top tiers. Hancock is dangerous because of her hax working. But that's the thing, if her hax works. Hell, her hax didn't even work against Pre-TS Luffy...
Anyway, Doflamingo was already compared to Hancock by Kuzan and he was simply portrayed stronger than her. He's also likely stronger than Croc as well. Heck, he's basically another part of Crocodile - similar crew structure (except his crew being better), both wanted to rule over a kingdom (except Doffy was successful) and so on.
You think Jack and Cracker come close to that?