The LSL games are great up through 7. 1, 2, 3, with a walkthrough handy, 5 is during the weird super-VGA/mouse interface switch happened and it's very easy, 6 is probably the best of the series, and 7 is decent. Don't even bother thinking about the rest--the college one, the hollywood one, or the dull recent entries.

Similar trajectory for most other Sierra franchises. The ega ones are nice but inscrutable and very die to die and almost as easy to enter a fail state but not find out about it until hours later, so walkthrough. The VGA ones and mouse interface ones tend to be softer and more forgiving, and the last entries were as the company and genre was failing.

King's Quest 1, 2, 3, 4 (ega era) 5 good but fairly easy (with a few soft-locks), 6 great, 7 pretty unimpressive, after that don't bother. Fans made some quite good re-makes of KQ1, 2, and 3 as well.

Space Quest 1, 2 (ega era) 3 (best one, but still ega) 4 for my money is when the wheels start to fall off, but the humor is still pretty good, 5 I recall being better than 4, and 6 is a mess.

Police Quest 1 good, 2 (ega era) great, 3 is where the decline sets in, but still good, 4 is just insanity.

Hero's Quest/Quest for Glory: Everything great except for 5. 1 wonderful, 2 even better, 3 quite good but vga and mouse, 4 wonderful but hugely buggy, 5 trash.

Those are the major ones, but then there's the others but similar:

Gold Rush, ega, good. Black Cauldron, ega, not very good Codename ICEMAN: ega, good, punctuated by bouts of awful Colonel's Bequest/Dagger of Amon Ra : both great Conquests of Camelot/Conquests of the Longbow: both great EcoQuest: not very good Freddy Pharkas, Frontier Pharmacist: decent, basically another Leisure Suit Larry Gabriel Knight: 1 good, 2 great, 3 okay but a departure, basically a different game

Then there's the oddballs that don't fit among those already listed:

Manhunter: New York and Manhunter: San Francisco: WEIRD, but kind of good. Not exactly point-n-click adventures, with arcade sequences. A lot of bizarre imagery and inscrutable puzzles. Rise of the Dragon, Heart of China, Willy Beamish: Timed events, bizarre puzzles, occasionally arcade. Not my cup of tea, but some people loved em. Phantasmagoria/Shivers: FMV puzzle games, akin to The 7th Guest. Betrayal at Krondor: Actually kind of great, but long, Betrayal at Antara: crap.

I think that covers most of what you'd be looking at, in Sierra adventure. I left out stuff like Gobliiins and INCA and the very early cga/apple adventure stuff and the garbage sports shovelware and decent but very decent games like Red Baron.

Overall, I'd play KQ1-6, SQ1-5, LSL1-6, QfG1-4, Police Quest1-4, Gold Rush, Colonel's Bequest, Dagger of Amon Ra, both Conquests, Gabriel Knights 1-2, and if you have time and life left over or itch for an RPG between adventure games, Betrayal at Krondor.

Then there's the old LucasArts catalogue...