Lydia's Heart is a game in the class of Anchorhead, Mulldoon Legacy, Curses!, and Worlds Apart in terms of size and story. To see the size of the game, check out the provided map, and realize that 90% of the rooms have their own detailed puzzle.
First, the story. You play a young girl at a southern motel who is entrapped in the mysterious plottings of a cult. You must find a way to escape their clutches. There are twelve or more NPC's, each of which can be asked numerous questions. The twelve NPCS's are mostly static, but later they move about a bit. The workings of the cult are explored in great detail, both at the motel and other locations.
As for puzzles, they are very, very difficult. This is the same author as Not Just an Ordinary Ballerina, which had very difficult puzzles as well. As an example, there are several locks in the game, which are opened in three or four different ways, two of which are almost never done in IF games. Items must be gathered from far away and assembled into one whole. Characters must be encouraged to move. And some just completely improbable actions must be taken.
However, I took a simple approach; I would just go as far as I could without getting frustrated, then start consulting the hints. The hint system is AMAZING. Just get as many hints as you need. Don't feel bad about it! The author intended this game as more of a story than just a puzzle fest; by consulting the first few hints for each puzzle, you're just making the level of difficulty low enough that the puzzles are still fun, but the story can still progress.
Several reviewers complained about mazes, but they don't realize that sometimes mazes are fun. The author allows you to bypass them with magic words, but then people feel mad about missing 100% completion. I subscribe to a different view; I love stories and settings, and I would rather skip all puzzles in a game to get a good story. Puzzles are fun, but they aren't the reason I play IF (except for Ad Verbum and Praser 5).
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.
Building is a real gem. I enjoyed playing this game on and off over the course of a few months. It is a medium-large size adventure game like Zork or Curses!, but set in a sort of post-apocalyptic office building.
The game has enormous attention to detail; the game's vocabulary is about 2000 words, and the number of in game messages is about 2000 as well.
This attention to detail becomes a bit too much at times, with descriptions that are over packed with words. Many of the puzzles depend on clues hidden in the middle of large paragraphs.
The game contains more red herrings than any other game I have seen.
In the end, after seeing some of the author's reviews here and his blog (the author is AmberShards), I wonder if the game is partially autobiographical. The author and the PC hate conformity, and fight against perceived oppression.
Curses is the first game I think of when I think of interactive fiction, together with Anchorhead. Sprawling, light-hearted, with a compelling backstory and cast of supporting characters.
For me, the beauty of the game is in the development of the plot, with a continually increasing sense of wonder. Another wonderful aspect is the open sandbox feel; this is a very non-linear game.
Although the game is very difficult (I've played through it three times, and had to resort to a walkthrough every time), there are so many puzzles that you will still solve quite a few on your own. Many puzzles have multiple solutions, or can be bypassed completely.
*Amusing things: There are three characters that have interesting reactions to all ten of the (Spoiler - click to show)rods. Those characters are (Spoiler - click to show)yourself, the knight, and Austin.