No contest. The Truman Show easily, every day of the week and twice on Sunday. It's not even close.
It's been noted in this thread already, Forest Gump is just baby boomer porn, while The Truman Show is entertaining, intellectually stimulating and still relevant to this day, not just in regards to reality programming, but with the continued rise in social media being used in an increadingly aggressive and exploitative way.
Although wouldn't be surprised if there is a cynical cashgrab done if there is a piush one day to make a Forest Gump sequel, with Hanks sitting on the porch talking to his grandson, recounting home he was on hand to stop the Reagan assassination attempt, gave running tips to Carl Lewis, being the brains behind new Coke that someone else then loses the recipe which then becomes a disaster, is in Berlin for the fall of the Wall, accidently makes a move that inspires the creation of the Macarina or twerking or something, is on a chat show with Kurt Cobain, on a tour of the White House happens to interupt Monica and Bill, stumbles onto the set of Titanic, is sight seeing in NY during 9/11 and of course lends a helping hand before being made out to be a national hero all over again by dubya Bush, makes a gazillion more dollars from the dot com boom, inspires Obama to run for office and then is wins Trump's celebrity apprentice, before grendpa Gump gently nodding off on the porch, passing away in his sleep as the feather floats away, all set to a soundtrack of 80s, 90's and 00's songs.