I’ve got to say, I have no idea, but my shot in the dark would be a toss-up between The Silent Cartographer and 343 Guilty Spark. The latter in Combat Evolved has a lot of implied lushness, but it’s not all there. I’m really looking forward to spending a bit more time in that swamp than usual, personally. That is, however, a very small portion of the level, which is why it’s debatable with The Silent Cartographer. In my opinion, Cartographer will really benefit from its draw distances being a lot less implied and more real. It’ll also really benefit from the lush greenery that’s been added - as we’ve already seen at E3. I had also considered the level Halo. It’s got a lot of huge open draw distances with tons of implied landscape. Get out of the escape pod at the outset of the level and start crossing the bridge, and look down and left. That’s not pretty to look at exactly, right? But you can tell it’s supposed to be. I think Anniversary will upgrade that to be much more solid throughout the level. So it’s really a toss-up between these three levels - mainly the first two - for me.
As for which levels will benefit least… Probably either Pillar of Autumn or Truth & Reconciliation. Both are pretty fun - if simple - levels, but regardless of how good they look in 2011, they’ll still just be claustrophobic ship levels. You can’t change that about them, and it leaves little room for actual visual upgrade.