I honestly don't believe Shanks is the Emperor that Law wants to ally against. Though I will say that the points made about Luffy seeing it as a chance to challenge and surpass Shanks has merit. I just don't think it particularly likely.
I wouldn't have pegged Blackbeard either, since he's been gathering all the hallmarks of an Evil Counterpart boss with the Straw Hats. A fight like that might be best saved for when Jimbei and Luffy's tenth join up and make Luffy's ideal crew. Though the point above by Saturnchild does make me wonder. What if they don't actually beat Blackbeard, but merely "de-throne" him as Emperor? They beat him, and take his territory, but he gets away to fight another day and now, with no Empire, he simply continues sailing forward like Luffy is currently. That I could actually see as it has the potential to make Blackbeard a consistent menace to the Straw Hats and plot as he continues his own journey to Raftel.
Kaidou is next on the list of likelihood to me. He's been given little overall development, but short of a physical appearance neither has Big Mom. He's been pegged as a crusher of would-be Pirate Kings with Moria. Also, and this is more personal theory, I think he has or has done something to Law's crew. Law's been conspicuously alone throughout this arc. Outside of the obvious possibility of the Hearts just waiting off to the side somewhere for their captain, I wonder if something hasn't happened to them somewhere along the way and Law's out for rescue and/or revenge. A point was made earlier that Luffy doesn't go for useless fights, which would put Kaidou lower on the list since Luffy has nothing personal with him. On the other hand, as the Baratie arc showed, Luffy does acknowledge that there are rivals he'll have to take on. So the question is would Luffy regard pre-emptively striking at a potential foe he's certain to have to take on eventually anyway as a useless fight or not?
Finally, Big Mom. She strikes me as the single most likely. She's the most recently introduced, has given Luffy a personal reason to want to face her, and can be presumed to be in close proximity. We've also met some of her crew, making her, despite what I said about Kaidou, the more established Emperor between the two. The major point against is that Oda made a point of not telling us which Emperor Law wanted. This points more towards Kaidou to me but also could still go to Big Mom because I could see Oda avoid mentioning the name to create suspense (which, as can be testified by all the debate going on here, has worked). Finally, Big Mom is directly tied to Jimbei joining the Straw Hats, as he's serving under her and needs to break off ties before he can join anyone else.
So, in summation, I think Kaidou or Big Mom are the most likely two Emperors we'll meet. Both for the reasons stated above and, from a storytelling perspective, I would think that using relatively "minor" characters as warmups / lead-ins to the major potential battles with Shanks and Whitebeard seems like the way to go. Ultimately, the question of whether it's Kaidou or Big Mom (assuming Law intends to go after whichever one it is immediately) banks on one question: How soon does Oda want to bring Jimbei into the Straw Hats?
Of course, that's just my opinion I can't be wrong.
Now to see how many posts got in while I was typing all this.