The golden age of text adventures was the 1980s, not the 1990s or beyond. However, that’s the ‘Model-T Ford’ era of text adventures. Computers were slow, storage was on cassettes or floppy disks (“What’s that?”, I hear people asking) and RAM was limited to around 48k. Two-word parsers were the norm, along with guess-the-verb, sudden death, getting into unwinnable situations without knowing it, no UNDO and so on. Despite that, there were some great games. Puzzles ruled.
For me, there are no “must play” games, just play as many as you can. I cut my teeth on Scott Adams, Brian Howarth, magazine type-ins, the excellent SoftSide Adventure of the Month series and the early Sierra On-line games (i.e. everything before King’s Quest).
I had a real soft spot for the illustrated adventures. Games like Sands of Egypt, Transylvania, Gruds in Space, The Neverending Story and Dark Crystal spring to mind.
You must also remember that the UK scene was completely different to the US scene. Level 9 and Magnetic Scrolls have already been mentioned, but there was a huge homebrew market with games written using The Quill, PAWS and GAC. Many of these were complete and utter crap, but there were also some real gems and some were very funny.
Cast your net wide and you’re sure to catch some gems. As already mentioned by @severedhand, Jason Dyer’s Renga in Blue is a great guide.