Who's-Who, high/high-extreme diff.

Any of the Tobi Roppo could beat Vergo (probably I haven’t really seen them fight yet)

Black Maria, Page One & Sasaki would lose. Ulti, Who's-Who & X Drake would win.

Do we still count drake tho?

Technically not. I only included him since he was part of the Tobiroppo at the beginning of Wano.

Doflamingo by his own count is somewhat below Jack. Who's Who is Jack's inferior by a slight degree too. At best Who's Who is around Doflamingo's level and at worst a bit below it. Given the Beast Pirates rank people puerly off strength and no other factors and that Who's Who is not one of the top 3 but the strongest of those below it.

Even though it's weird to think about Doflamingo got power crept quickly, just like Katakuri. Luffy has to keep going to the top so he could fight the Yonko next

I'd say Doflamingo > Jack > Who's-Who

Why would Kaido send someone weaker than Doflamingo to threaten him and why would Doflamingo care about a weaker fighter than himself being sent?

One of the reasons is because Kaidou is also the one backing him up in a similar way to how Orochi had it among other reasons. Doflamingo is a powerful adversary and sending King & Queen to Doflamingo would be somewhat useless since they would be needed at Wano as Queen keeps Udon Prison in check and King is Kaidou's right hand man. Jack was the strongest one he could send to keep Doflamingo in check. It would also be a tactically better idea since he knew it was a ship with Marines that was carrying him though Kaidou didn't know who was on it. He wouldn't want to risk sending his two strongest fighters but sent Jack to at least threaten Doflamingo. In the case that there were powerful marines (which there were), it would also be efficient since Jack is part Fish-Man and can get out of there if necessary and the Marines wouldn't waste their time with him. I do know King can also fly out of there but again, he's Kaidou's right hand man. Jack was a good decision to threaten Doflamingo as he also has Kaidou backing him up and Doflamingo wouldn't mess with one of his All-Stars. Doflamingo was also incapacitated by this time so it wouldn't matter if Jack was stronger than him or not. TLDR: Jack was tactically the best choice to threaten Doflamingo since he can escape if necessary.

I'd argue his tactically best choice would have been someone smarter and quicker then, such as for example Dias X Drake, a Tobiropo Supernova with knowledge of Marine Ships and who needs to prove himself.

I also might be wrong because I can't quite remember where I heard this, be it a small line in Dress Rosa or a qna with Oda but I do think Queen was sent to Dressrosa in the past. And as another point for why Queen may be a good if not better choice to save Doffy would be that he can be invisible even to Haki, and he can infect strong people with diseases Haki can't prevent or counteract.

Tdlr. Jack was a good choice, but Queen or Dias Drake were both equally as good if not better options.

And as a complete sidenote, Luffy in Gear 4th high digged Doflamingo, and with support from Nami with more time he managed to do the same to Charlotte Cracker. A fight Luffy would've lost if not for Nami countering the advantage he had over Luffy, which implies that Charlotte Cracker is superior to Doflamingo and Jack. And honestly that might be true, as Perospero and Jack at Wano got beat by opponents on the same level of power. But at this point I'm not even arguing about where he is but moreso just thinking aloud