Okay, a potentially confusing one, but I've tried to keep it to the more familiar current-and-former DC Green Lanterns from Earth.
So the question is, which is the first one you read about in a comic book? Any other candidates (from Earth or from other parts of space) can be included under "other", but please specify who in a separate post.

Choices listed here are:
* Golden Age Alan Scott (Green Lantern introduced in 1940; later went by "Sentinal")
* Hal Jordan (Silver Age Green Lantern first introduced in 1959)
* Guy Gardner (first "backup" Green Lantern, introduced 1968; later went by "Warrior" and was a Red Lantern)
* John Stewart (backup Green Lantern introduced in 1971; also was a Darkstar)
* Kyle Rayner (introduced in 1994 when the GLC was destroyed; later went by "Ion")
* Simon Baz (introduced in 2012 during the New 52)
* Jessica Cruz (introduced in 2013; originally known as "Power Ring")
* Jennifer-Lynn Hayden (initially Jade, daughter of Alan Scott; given GL ring by Kyle Rayner in 1998)
* animated Hal Jordan (seen in comic book based on the animated series published in 2011-2013)
* New 52 version of Alan Scott (reimagined Alan from the series Earth 2; introduced 2012)
* other (please specify who/when in a separate post)

NOTE: There are two "Hal Jordan" listings; one is for the Silver Age+ version first introduced in Showcase #22 (1959), while the other is for the version of Hal Jordan from "Green Lantern: The Animated Series" which had a comic book based on that show from 2011-2013.

ALSO NOTE: there are two "Alan Scott" listings; one is for the more traditional Golden Age Green Lantern from 1940-2011, while the other is a more modern version introduced in the New 52 series Earth 2.