The Hitchhiker's Guide text-based game is, IIRC, most well known for having what is arguably the single most difficult puzzle in text game history. "Arguably" because the solution is only ridiculously difficult if you've never read the book. When you first find the babelfish, you must type the following command: "Put fish in ear."
Based on your whether or not your definition of "most difficult" includes the above, the second (or first) most difficult puzzle in text-based games is the Sorcerer (another Zork-based Infocom game) time-traveling gas mine puzzle. I can barely understand most walkthroughs of that goddamn thing, much less figure it out on my own.
BTW, if you're playing any Infocom games, check out
this site for the original Infocom "Invisiclues" booklets. Basically, these were little booklets that revealed the answers to the questions asked when you used a reappearing-ink pen on the answer area. The best part? To avoid making the questions themselves spoilers, there are a lot of just totally invalid, off-the-wall questions, with equally bizarre answers. And some of them are just wonderfully silly; for example, in the one for Hitchhiker's, there's the following:
Q:
"What's that bulldozer doing in front of my house?"A:
"The backstroke?"A great read, even if you're not playing these games. ^_^
