Of the witnesses, I suppose that I think Mrs. Salmon's evidence will be most convincing, followed by that of Mr. Wheeler.

The reason for this is that their evidence is more likely to show that Adams actually killed Parker.  Mrs. Salmon's in particular, shows Adams holding the murder weapon and wearing gloves.  This is much more convincing than just seeing him near the home or even leaving the home.

However, the most convincing evidence is the fact that Adams has an identical twin.  We know one of them must have been the killer, but there is no way to say which.