Hal Jordan has been said to have had THREE best friends. First, there was Tom Kalmaku, Hal's mechanic and confidant, and the closest thing he's ever had to a true sidekick, from his early days as a Green Lantern.
In current continuity, Hal's best friend is Oliver Queen, the Green Arrow. Together, Hal and Ollie tend to be paragons of stereotypical macho bonding, but Ollie also, often serves as a counter-point to Hal from a sociological standpoint.
Then there was Barry Allen, the Flash. Hal and Barry's friendship seems to have grown out of the fact that, during a team-up, their powers were remarkably compatible and effective together (see JLA: Liberty and Justice for a perfect example of this). Despite the fact that Hal and Barry had very different personalities, they became good friends, in a much more lighthearted friendship than that between Hal and Ollie.
Barry Allen, unfortunately, died during the first Crisis, on Infinite Earths (1985) -- though DC Comics is now taking steps to bring him back. Needless to say, Hal took Barry's death pretty hard, especially since Hal wasn't even around -- heck, wasn't even a super-hero -- when Barry died.
Many new Green Lantern fans, while researching GL history, have been asking, "Where was Hal Jordan during Crisis?" Not an unusual question, because Hal wasn't a Green Lantern at that time. Here's an overview of what happened during that time, and what led to Hal's unhappy discovery of Barry's death.
Just prior to Crisis, Hal's longtime boss and girlfriend, Carol Ferris, got fed up with Hal leaving all the time to go play super-hero, so she gave him an ultimatum: Either the ring went, or she went. Despite the fact that he LOVED being a Green Lantern, Hal actually chose to give up the ring. He quit the Green Lantern Corps, and John Stewart was made the head GL of space sector 2814. But when Hal quit the Corps, Carol Ferris transformed into a very nasty version of Star Sapphire -- and remained that way for years. She abandoned Hal and left Earth, right when Crisis began.
Determined to help during Crisis, Hal contacted John Stewart and set into motion a series of events that eventually brought him back into the Green Lantern Corps, and back to his status as a Green Lantern.
Most Corps members spent Crisis in space, dealing with various catastrophes on a cosmic scale. By the end of Crisis, many Green Lanterns had died, lost their planets, and lost their home sectors.
Hal had lost Carol Ferris and lost Tomar-Re, his closest friend in the Green Lantern Corps. Despite that, he felt thrilled to once again be a Green Lantern and happy that Crisis was over. He had no idea what had happened back home on Earth, but he found out soon enough, during a euphoric flight through space. Here's what happened (from Green Lantern #200, 1985):
"-- HEY! I'm a Green Lantern! I'm glad I went through all that hell -- now! Now that I really know how much I love this life, I'll never think about giving it up again! I -- huh?"
The worst thing about this sequence is, that's not even, really Barry. In one of the meanest stunts a super-villain has ever pulled on a super-hero, Barry Allen here is actually a construct conjured up by Hal's enemy, super-telepath Hector Hammond. Hammond had been mentally spying on all of Earth's heroes during Crisis, and sent the image of Barry to Hal solely to torment Hal.
Mean. Just mean.