For me, it would have to be The Daleks. I would love to see what the Mill etc made of Skaro and that shot where the Doctor looks through the binoculars at the Dalek city on Skaro. I have CGI'd that in my head many times, but I would love to see it in HD
Although, in the feature on The Chase DVD where Raymond Cusick's talking to the NuWho effects guys, they cite the Dalek city as an example of where models look much better than CGI - more ethereal and otherworldly.