Prior to Geoff Johns taking over the series in the mid 2000s, I would have answered this with John Stewart. Primarily, it is because of the influence of the Justice League/Justice League Unlimited animated series, where John Stewart was a very prominent character in the show.
However, before that, I would have said Hal Jordan, as he was the only Green Lantern I was familiar with. The only thing I know about Guy Gardner was that he was punched in the face by Batman. As for Kyle Ryner, my only exposure to him was in that one animated episode on Superman the animated series.
After Geoff Johns's illustrious, and Green Lantern mythology altering run, hands down it is Hal Jordan for me.