A thread in intfiction.org attempts to establish a baseline of the finest pieces of interactive fiction available.
My ten favorites, in probable publishing order:
Zork II: The first Infocom game I ever played (back in 1984), well ahead of the first part of the trilogy. Took a couple of years to finish. Despite its shortcomings (never understood the bank puzzle), this treasure hunt with a persistent enemy is still an entertaining game.
Planetfall: The first Infocom game with a plot. With the bonus of Floyd the droid, the first properly realized interactive NPC (who even responds to meta-commands).
Wishbringer: A beautifully written beginners game that had me stumped for a long while. Platypuses, transformed town, multiple solutions to problems. What’s there not to like.
Guild of Thieves: While The Pawn was more of a technology demo to show Infocom that Magnetic Scrolls were a real competitor, their second game was a much better product. A relentless treasure hunt through a quasi-medieval milieu was perhaps cliched, but an impressively put together collection of puzzles.
Unnkulian Unventure: The first new generation game that I played. A game that proved that there’s still life in the genre even though the commercial publishers are dead. Humorous and complex. A perfect showcase for TADS, the language and virtual machine that allowed development of highly complex games.
Curses: The Inform language and compiler began intimately tied to Curses. A massive game that mixes in puzzles of variable quality and difficulty. Immensely enjoyable, but packs a steep learning curve.
Photopia: Short, pointful and emotional.
Anchorhead: Long, pointful and powerful. Finest horror game I have played. So good that I actually crafted a mostly functional Call of Cthulhu- scenario out of the plot.
Lost Pig: The most recent entry on the list is yet another impeccably written game. The point-of-view of a not so smart protagonist is well realized in an avalanche of appropriate responses to most commands.
And looking at the other respondents’ lists, I see that there’s tons and tons of good interactive fiction to be picked up and enjoyed. Hence #10 remains open.