I still think of Vermithrax as the definitive cinematic dragon. For being over 30 years old, the special effects still hold up well. Better than, say the CGI of the late 90s or even some of the early 00s.
I'd have to go with Keith Parkinson's dragons for some of my favorites:
This is the first picture of a dragon I remember seeing, when my mother read me the Hobbit as a 4-year old boy. There are better pics but this one captured my imagination and sparked something that lives on to this day.