Zork is the most famous adventure game, although it was not the first. This version contains much of the three Infocom Zork games which were developed later.
Zork is a large puzzle-heavy exploration game. It has inventory limits, a timer of sorts (the light in your lamp), and it has several unfair puzzles (depending on the version you play, some important in-game clues can be omitted). The exits in the rooms work in a non-symmetric way, so going north and then south might bring you back to the wrong place.
I found that mapping out the entire game myself was very helpful. Instead of drawing a map, I just made a numbered list in the notes section of Frotz of all the rooms and their exits. That alone let me get much farther than I did 5 years ago.
I used walkthroughs after getting about half of the points, but the version on IFDB contained a fatal bug preventing me from completing the endgame. I found another version online that ran slower but which allowed me to complete the ending.
The game gets better the further you get. The 'hidden' areas are really fun, and I was surprised how huge this game really is. It makes sense that it was split into 5 games later.