Markakis, Holliday, Bay, Upton for me.
I'm sure Bay is younger than he seems to me as someone who remembers him as a Padres prospect (Never fear, Giles was sure worth it, ugh), but I can't shake the feeling that in a long-term keeper he has to be downgraded as a guy who isn't going to get better than he is right now.
That said, you've got to have legitimate concerns the same is true of Holliday never again reaching his Colorado numbers or Upton being a power threat again, so to a certain extent I think you can reasonably differ on which of those three you're highest and lowest on.
Markakis seems like the surest bet here to me. I just don't see the downside, young guy who's already producing at the level of the rest of the guys listed and could get even better.
By the way, I intentionally avoided looking at the other responses before posting this so as I hit post I'm now about to be really curious whether everyone thinks my order is stupid or if I'm close to the consensus.