Rosewood or Ebony would likely have the best wear characteristics for the keycaps, partly because they are harder and denser than the others.
Rosewood's oil content means it wouldn't show even when the finish begins to wear off - if anything becoming more polished (not sure how they got the finish to stick to it in the first place).
Ebony will look good with or without the finish even though it looks like one of the Asian varieties.
The others would all eventually start to stain once the finish wore through, though most likely the Cherry would look awesome in person as it usually has a wonderful chatoyancy to the grain that shimmers with changes in viewing angle and it naturally darkens substantially with age.
