She certainly thinks so!
"If you are to pick only a single (I know it is hard) game that best represents your achievement and your career as a game designer, which game is it? Why?
If I could only pick one game, I would pick Phantasmagoria, as I enjoyed working on it immensely and it was so very challenging (and I love to be challenged!). However, in my heart, I will always love the King's Quest series and, especially, King's Quest, I since it was the game that really 'made' Sierra On-Line."
http://www.adventureclassicgaming.com/index.php/site/interviews/198/
I would tend to agree with this. I'm of the opinion that an artist's first works of their careers or of new genres are the most indicative of their real artistic style.
Phantasmagoria is Williams' darkest game by far, but certainly not her only dark game (she also wrote the Laura Bow series). Her real talent is in making likable, pitiable characters no matter what genre or tone they are placed in.

For anyone who hasn't played the Laura Bow games, they came withe King's Quest "7 million sold" big box collection, the intro to Laura Bow Colonel's Bequest goes like this:
There is a will that takes the whole screen,while the title of the game is above it, the camera is by the right side facing it.
Then a hand with a feather pen comes down and starts writing on the bill.
(Ominous music)
It finishes writing the bill,it goes off screen.
Then...
(A big thud sound,presentation mystery music)
A big knife comes down with its blade facing the bill, when the knife hits the bill, bloods is dripping from it. The title screen is tainted with blood and begins dripping blood too.
After that horror presentation, we are presented to a joyful 30's(or 20's,I don't know much about old American music) music which introduces us to the character,wtf.This thing was ahead of its time and creepy.
The rest of the game depended whether you chickened out(you could avoid everything,discover nothing, but made it out alive) or not. If you didn't, lets say you had about NO chance of surviving especially when at the very beginning of the game, right next where you start, THERE IS A FREAKING DEATH SEQUENCE.
START GAME
WALK LEFT
GO NORTH
TRY EVERYTHING THERE
DIE
When that happened, I really wanted to play any other horror titles made by Roberta. Phantasmagoria is a great cinematic adventure but I would not call it defining game, I'm between Laura Bow and King's Quest V.
BTW, I bought Phantasmagoria off GOG. I recommend it.