Some of my favorites are Birdland, Counterfeit Monkey and Midnight, Swordfight. I also really loved the Exceptional Story "Codename: Sugarplum" in Fallen London, but that's a lot less accessible.
I just want to thank you for introducing me to Birdland; I've played that and am going through the rest of Brenden Hennessy's works now. They're really quite fun!
I've played Counterfeit Monkey before and I +1 that recommendation; I think it's got some very clever word puzzles.
Glad to see someone else likes their work :)
I really enjoy Brendan's mix of humor and more serious themes. I can't wait to play more bell park if it comes out :)
Anchorhead will always rank very high for me. I played through the Z-machine version, and while I've purchased the Glulx version, I haven't played through it yet. I hope to soon, though. The game has great atmosphere.
On a somewhat related note, I've found that Paul O'Brian's reviews of IF Comp games over the years are uniformly excellent, and may help find some new favorites. He tailed off from reviews a while back, but now has a new site called Inventory which is dedicated to IF as well, and worth taking a look at. He's copying over his older comp reviews as well as collecting various others he's done.
I'll go first. My favourite interactive fictions are Magium and both Eldrum games. I'm also liking a lot of the interactive fiction that Choice of Games and Hosted Games have, but I'm quite new to those. I'm currently reading Wayhaven from Hosted Games and its a good book where you're a detective and there is a lot of supernatural stuff in the book like vampires and werewolves. I highly reccomend it.