The Yonkou That Law Talked aAbout

was thinking at first that it was big mom, but think about it from luffy's reaction you'd think he of all people would be more enthusiastic

i really dont think that it is shanks, you can pretty much tell that whenever we see shank's true power it will be towards the end of the series

it definitely isnt teach becasue of the power he's gained, oda will wait a while before there's any more conflict with him

last but not least, kaidon(i think thats how you spell it) he's a relatively unknown character if it was any of the other three, believe me it would have been confirmed already, so he's the only logical choice

It´s Kaido(u) ^^

And it could be, but I don´t necessarily think any of the others bar him would have been confirmed till now. The thing is, Oda might just want to make us guess and generate suspense. The logical choice would be Big Mam, because the SHs aren´t strong enough - even with Law - to have 2 Yonkous as enemies (but that doesn´t mean Luffy ain´t crazy enough to pull it off!).
I think he´d take Shanks´ crew alone, without any help, so I´d count them out. I don´t think it´s BB either, since he´s saved for the end...

But since this is Oda, I guess we can expect some surprises :)