Gee Whiz. Like many others, I'll have to factor in the remakes as well as the canonical ones. Lowest to highest:
Romancing the Throne (KQ2 original): Some puzzles were a little too easy and silly. Some of them were plain illogical, and loose ends galore. It probably looked better on the developer's table than in practice.
Absence Makes the Heart Go Yonder: Cedric, surprisingly, isn't much of a factor here. The pointless ways to die, the notorious amounts of dead ends, and the creaky audio are more grating. Still has to be thanked for the graphical interface.
Quest for the Crown (original): Seems a little unfair to rank it so low, as it started the whole thing, but it had more than a few hair-pulling Guide Dang Its and the others were able to build a story from the roots of it. Still, the puzzles made more sense in the first than the second.
To Heir is to Human (original): OK, it's not a King's Quest unless there's a pointless way to fall to your death. They seemed to be getting on their feet story-wise, and the puzzles made sense. The only issue here is the waiting...and more waiting. NICE Easter Egg about the...ahem...proof of identity Alex ends up providing...(and how many fangirls here wouldn't mind the view?

)
To Heir is to Human (Remake): Nicer interface, and GREAT additions like Mordak's cameo and Rosella's farewell to Graham (along with Josh Mandel officially voicing Graham in more fan games than canon ones!), but still the issues with waiting and waiting are there.
Perils of Rosella: A little more strength on the story, the interface is smoother. Still a few really harsh Guide Dang Its (the bridle is VERY tricky), but the strengths make up for it.
Quest for the Crown (AGD remake): It's a faithful adaptation, smooths out the wrinkles, adds an couple Easter eggs, and is generally nice to play, but it's still the first game without some of the awesome the later games get.
The Princeless Bride: The upsides are no dead-ends, logical puzzles, a great sense of humor, solid voice acting, and Valanice
finally getting to step up to the plate. The downside is a really garish art style that looked better on Space Quest 6. Odd as this sounds, it also was just a hair too easy, like it was made for younger players than KQ6 was.
Romancing the Stones (AGD Remake KQ2): This is what fanfic SHOULD be in every way, shape, and form. My hats off to them. It takes the one I thought was the most annoying entry and actually arranges it to
make sense! Wicked twists with the Werewolf vs Vampire plotline, marvelous nods and tie-ins to the other entries. The Air Gem tests were
AMAZING. The Lewendowskis' music was worth downloading all on its own merits. Everyone gets fleshing out, the loose ends get explained, and the curse at end...*shudder*
Heir Today: Gone Tomorrow: Holy Crap! Is that Robbie "The Beast" Benson voicing the lead? There's a reason the dude's typecast...he's just too good at playing that type. And...That's Tony Jay and Don Messick! SWEET! The music is excellent, the artwork top-notch, the interface smooth, the sense of humor and the heavy-weight stuff blend instead of grate on one another. Cassima kicks butt and...Did Alex just make Death cry? Oh, this is AWESOME!