Loom is fantastic, and is a must-play. There are numerous VGA versions, so don't let the EGA graphics scare you off. There are also, I think, some voiced versions out there.
Grim Fandango is also a must.
Sanitarium is all right, but pretty maudlin and suffers from a character who moves way too slowly, puzzles that are far too easy, and--in an intersection of these two flaws--too many "go from A to B back to A" sequences (i.e., "puzzles" that are obvious on their face but with a solution that is always the farthest possible point from the obstacle). I'm not sure, actually, that I would recommend it. It has some grim/surreal stuff that seemed cool when I was 16 (I think that's when I played it), but I'm not sure how well it would hold up in that regard now.
The Dig is boring and very atypical for a Lucas Arts game. Flat, cliche characters, lifeless environments, heavy-handed and preachy story.
Both of the Indiana Jones games are quite fun.
I enjoyed the first Broken Sword game quite a bit. They got progressively worse, but the visuals of the first are nice and some of the puzzles are clever. Lots of stupid puzzle-solving, though (I believe that Old Man Murray did an article on this, or possible it was Gabriel Knight).
Gabriel Knight 1 is good, although it has lots of Sierra idiocy in it: if you miss an item that's not obvious, you lose the game hours later with no recourse to fix it, and at least one puzzle seemed impossible to solve without refering to a walkthrough (but maybe I missed something -- I'm thinking of the voodoo code / crypt puzzle here). The second one is a godawful interactive movie, and the third is a godawfuller 3D game.
Space Quest has inferior Lucas Arts style humor with all the Sierra flaws. I wouldn't bother with it, unless you're desperate.
Quest for Glory 1, 3, and 4 are fantastic, don't have much Sierra nonsense, are funny and open to replay. I highly recommend them. The second one is unplayable in my opinion. Both the original EGA version and the fan-made VGA remake involve an endless series of twisting passages, all alike, as the VERY FIRST THING the player must confront -- and this maze of alleyways is always in your way for the first few hours of play. The VGA remake is poorly designed in many regards, with wildly fluctuating difficulty and graphics that range from good to lousy. (The walk cycle, in particular, stands out as terrible.)
Full Throttle is a lot of fun, but short.
An old and somewhat obscure one is Dragonsphere. My experience with the game is idiosyncratic: I played a demo of it as a youngish kid (I think it was roughly contemporaneous with KQ5) and thought it looked awesome, then never saw it again till I was maybe 19. It seemed like very few people had heard of it, so I felt a certain proprietary attachment to it, combined with nostalgia. All that said, I think it's pretty cool and has a nice story, though it suffers from some Sierra-style flaws (I think it's a Micropose game, though).
Conquests of the Longbow is a monumental Sierra game. It's not without its flaws, but at the end of the day it's worth playing.
The Longest Journey is okay. Its sequel, Dreamfall, is terrible.
Syberia, another "modern" adventure game, is very pretty and atmospheric, but is crippled by the protagonist's slow walking speed and the number of back-and-forth quests.
Then there are a handful of freeware ones, but I think those are discussed in another thread.
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. It's also over 10 years old, so I consider this abandoned.
