I think there are several angles to this question, such that I don't think there is a definitive answer.
The more important question is what is the longest game you Really enjoyed? I'd rate The Longest Journey tops here. Yeah, there was lots of conversation, but it was far more relevant to the game than say most of the stuff in the Myst games. Its entirely possible to invent puzzles that are nearly impossible to solve. I've run into lots of those too. I wrote a program to solve the God brick puzzle in Tex Murphy Pandora Directive. The story was compelling enough to make it worth while. Then there was another game with a circle wheel puzzle in an Aztec temple (I think) with marbles that took sixty something steps with a walk through to solve. I had no idea how to write a program to solve this in a reasonable amount of time. And this is fun? Then there are the time pressure puzzles that I truly detest. Try something and die, then try, try, try again. I can tolerate a few, if the story is good enough and it fits into the story well. Game developers would be well advised that most adventure gamers are not sadist/masochists and just want to have fun and enjoy the game. Many of the old school may have advanced math skills, but don't count on it if you want to sell to a broader crowd. Nobody can solve every problem heaved at society. Thats why I'd nominate MaG for a Nobel prize in game solving if one existed. Sometimes we ALL need help in something.
Which brings me to my nominee, Indiana Jones and the Fate of Atlantis. For more modern games Syberia gets second prize. And Road to India gets my nod for a fun game for those just starting out. I think its on sale at Adventure Company now. Try it if you haven't played it.
Albert
I would have to say The Longest Journey was the longest for me too. I like puzzles but I don't like being stuck for a long time when I am trying to consintrate on the story.
You made some very good points here. I like the puzzles in adventure games as well but my faverite has always been inventory bassed puzzles. I can figure out how to put things together to make something much quicker than other types. I hate puzzles where you have to run from screen to screen and move thing to put them together or timed puzzles. It's also nice to be rewared ehrn you get it right so you know it's done. I loved Pandora Directive because of the puzzle skip code it had. The story was so good I skipped the puzzles the first time through and then played it again doing all of them and not skipping any of them. I also remember that marble puzzle and I did that without a walkthroughand it took me about an hour. I think it was in Martin Mystere. There are also some hard ones in Nancy Drew White Wolf and they are random so a walkthrough won't help.