Well I'd rank Nelson above all the minor leaguers you listed. He is already a very talented prospect and playing at Coors will only help him. As for who I'd drop for him, either Kendrick or Sanchez. Though with all the middle infield prospects the Angels have, I'd most likely drop Kendrick.
Wood is looking likely to move from SS to 3B or OF, so that will make him less valuable. He is off to an amazing start, but since the Anges have Aybar, Callaspo, and Kendrick ... his bat should be moved.
McCann right now, since Mauer is no longer a prospect, is pretty much the top catching prospect in the minors. Has very good power potential and should hit for AVG. also. After the year is over though I think he will have strong competition from Neil Walker for top catching prospect.