I always loved adventures. And I like them long and involving, with good, difficult puzzles. This is a list of the best ones I played. I placed Infocom's games in last place, not because they're no good, but because they're usually not free and difficult to find.
1. Lydia's Heart by Jim Aikin (2007) Average member rating: (30 ratings)
Maze says:
The best of the bunch. Some aspects of the other adventures are better, but as a whole this one is unbeatable. It's got the most logic and story-driven puzzles I ever found, and a lot of warnings if you're going towards an unwinnable condition.
2. Curses by Graham Nelson (1993) Average member rating: (129 ratings)
Maze says:
Huge. Filled with puzzles. Deeply involving. Unfair. You will probably need some hints to complete it, but the experience is nonetheless very satisfying.
Rich and very difficult. With some logic/math puzzles that will get you crazy. In spite of being more an array of puzzles than a story, the first part will give you the creeps.
4. Slouching Towards Bedlam by Star Foster and Daniel Ravipinto (2003) Average member rating: (214 ratings)
Maze says:
The setting is fascinating, and this game will quickly drag you in. Not too difficult, and not exceptionally long, with many possible endings.
5. Varicella by Adam Cadre (1999) Average member rating: (128 ratings)
Maze says:
Not exactly an adventure, as it lasts only 100 moves, and you'll have to replay it many times to *win*. Nonetheless, if you play it with the right disposition, you'll be totally captured by it's rich complexity and depth, and it will become a *long* game due to all the time you'll spend replaying it.