Chex Quest duh, and it came with free cereal to boot.

Depends on what you're into:

King's Quest - fantasy with some humor
Space Quest - humorous sci fi
Police Quest - police procedural / detective
Quest for Glory - fantasy adventure/action/RPG hybrid

Honestly, they're all fantastic.

Titan Quest

#SPOILERS AHEAD#

That damn glass shard in Space quest!!! Even after I had to resort to a walkthrough which told me about the glass, I still couldn't find it. There is absolutely no hints and you have to type exactly the right thing.

Kings quest 2 wasn't too bad. I actually figured out the whole game without a walkthrough (except that saddle puzzle. I just killed the snake)
KQ 3 on the other hand, is an absolute bastard of a game. So many very specific commands are needed. Like that magic map which saves you so much time. Why the hell would I "look behind clothes" and expect to find a magic map.

Despite all that, I love all the quest games. I chose to play them in the chronological order they were released. Started with KQ1-3, then SQ1 etc. Gives a historical perspective to how the genre developed

Of the ones I have played very few hold up, but King's Quest VI does. Police Quest 4 feels like a 90's cop movie, which is kind of amusing. Most of the rest are just too boring or difficult today.

There are many I haven't played though, including the ENTIRE Quest for Glory series.

simon's quest :3

Space Quest 6 was fun when I tried it and it hadn't even aged horrendously.

Yeah I know it's silly how you have no way of ever knowing that the shard is there in the original version of Space Quest 1. In the VGA remake they at least show glass shards on the ground and make it sparkle but in the orginal I actually tested at least 12 different ways of getting the game to tell you there is a shard there and the only way to do it is to type in a very uncommon command that a average player wouldn't probably type, I think it was something like "examine ground around pod" or something similar, I don't remember but either way it was something no player would actually tell the game to do unless they knew something was in that area which we don't.

As for KQ 1+2 I actually found them to just be really cheap and annoying mainly due to some of the random crap like the thieving Dwarf, the Wizard, the Wolf that randomly appears in Grandma's house, and that damn Gnome but I never thought they were awful though, i look at the KQ's save for 6 a bit like playing Scott Adam's adventure game it was fun back in the day but looking at them now they really haven't aged well so I really don't recommend them to newer players. But that is understandable since the King's Quest series are often the very first games from Sierra to use new tech, the first game was the very first fully graphical "avatar based/3d" adventure game so much like the first text adventures there was going to be some growing pains as the series and the genre progressed, four was the first to use the new SCI0 engine, five was the first game to use the new point and click interface as well as CD roms and so on even if it did give us silly things like that damn Owl "Oh no, Graham it's a PO-sionus snake" and some really silly illogical puzzles like hitting the yeti with a pie..

Quest for Glory - Offers the most diverse play style with the most replay value since you have 3 different character types to start which you can customize before you begin and many of the puzzles have multiple ways to solve and unique ways for each class. Add a decent amount of humour and varied combat in each game.

King quest V. My serria's favorite game... you're gonna love the Owl :-).
ohh and the desert part !
No reall, you should try it.

The Quest for Glory series is the best. I like QFG2 the most, although it's the most dated so you might want to play the AGD remake for better graphics. In Quest for Glory 3 you need to pull the trigger of Chekhov's gun to defeat your final adversary as a wizard.