Tobirama is my bet.
My order is as follows.
1. Tobirama
2. Minato
3. Hiruzen
4. Hashirama
We know that so long as Minato has Kurama, he cannot be taken out. This gives him protection until he passes it on. I am also under the assumption that this is not a "quick tap" transfer, and will thus take an great deal of time.
My bet is that as Minato begins this process, Tobirama will Sacrifice himself to buy time for the next generation as he did before...possibly giving us insight into his demise. At this time we will probably gain access to his large scale Suiton to push Obito back and continue to pressure him....leaving behind the rest of the group in a one on one fashion. Knowing his Ninjutsu will be virtually useless, Tobirama will display the prowess of his taijutsu and Kenjutsu, but ultimately be destroyed by obito.
Following this, Minato will complete his Kurama transferal, and push Naruto aside to move him out of the way of an attack....as he disappears he'll give off some corny one liner like, "I'll leave the rest to you son!"
Around this time, we will likely see Obito evolve into the final form Juubi Jinjuriki (remember the Juubi is not in its final form). Upon so, virtually every attack will become useless including Senjutsu. Knowing that this has to be countered, Shikamaru will devise a strategy. However, knowing that the brains were targeted once before, they will inact the orders via Hiruzen who will act as the tactical overseer. Thus becoming the target for Obito to hit... however it will be a lure that will lead to the Hyuga Clan or some other technique trap to damage his chakra system to take away his invulnerable state (though his power will remain).
Madara already mentioned he wanted to keep Hashirama around which is something the other Hokage do not have so I'll predict he will go last...also as the strongest Hokage...he will likely go last (NOTE: The rest of the Hokage do not leave from weakest to strongest...only Hashirama).
That's my thought anyhow.
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