I think for me it was either The 39 Steps or John Carpenter's "In The Mouth of Madness", what film was it for you?

Tom Jones,tore down my Monkees posters that day and have been smitten ever since.

If i'm honest the first film I ever saw David Warner in was Teenage Mutant Hero Turtles 2: The Secret of the Ooze. However, I was but a child and as a teenager the first performance of his that greatly affected me was in Straw Dogs.  

It Was Nightwing For me, caught it on BBC1 late night around 1987, then Waxwork a few years later on vhs, then became a fan as David would pop up in loads of stuff and I would catch up on his previous work.

As for me , I think the first one was "Time bandits" , probably "Masada" also in the eighties... then "Straw dogs" . But as a matter of fact there has been "Holocaust" too in the seventies ! I became a fan later on, with "The Ballad of Cable Hogue" .