For me Vangelis is unique
A True artist who relies on instinct and isn't constrained by technique (perhaps only by what he can use to produce sound)

There will be another John williams. if you look closely at his music there are common threads throughout such as the influence of Stravinsky copland, or Holst. Vangelis seems to take different directions each time capturing the essense of sound and emotion.

I do think both are superb musicians but i believe that 200-300 years from Vangelis's music will still be here.