I haven't heard that Snow or Poison are going to be in this set, so that rules out some things. I think Zenith Chronicler is likely because it plays well in Commander. Personally I'm rooting for one of my pet cards, Phyrexian Walker. It's probably a dark horse but I think as zero-cost creatures go it's one of the fairest.
For a second I thought Phyrexian Dreadnought was a Phyrexian Construct before I remembered it's on the reserved list. Plus there's 0 chance they'd reprint that anyway!
My vote too is for Walker. Really cool classic card and another 0cmc for the bigger formats.