Gollum

Originally Posted by Legate of Amon Lanc

I can only quote Gandalf here:

I think this is quite clear. Gollum, as much as other halflings, was quite resistant to the effects of the Ring. And he probably was not very much nearer to becoming a Ringwraith than, let's say, Bilbo; or Frodo after the Morgul-wound.

Gollum held the Ring longer than they, & anyone else for that matter. The transformation from halfling to a green slimy creature seems to be a indication of the first major signs of a Ringwraith developing. Also there is the extra physical powers which Gollum now has, such as keen hearing & sharp eyes which light up even in the dark.