Strangely, I think that Blackadder series is actually the best portrayal WWI's ever had on television. Which is really goofy considering it's a sitcom...
There's lot of good material on WWI out there, but you kind of have to dig a bit deeper to get at it. If there's a war film made between 1920 and 1940, you can bet it's about WWI, for example-- Netflix has dozens of these and IMDB can help you locate them. You can start with, say, Hell's Angels, you've seen All Quiet... if you want to get very disturbed, take a look at Johnny Get Your Gun the 1971 version.
BTW, Flyboys the 2006 movie is ... acceptable, but not very historically accurate.
What I'd really love to see is a WWI movie documenting service for a tank crewman. Can you imagine serving in a Mark IV? Steel walls, moving over rough ground with no suspension to speak of, 120 degree temperatures, the noise and fumes of the diesel right next to you, the constant risk of fire, gas attacks, armor being pierced by a bullet, or having artillery leveled at you. Or simply breaking down and having to hoof it back to cover... It would be like a somehow-even-more-cramped version of Das Boot.