The use of "as" in "as fulfilling" means it is a comparison (A is "more fulfilling/less fulfilling/as (equally) fulfilling" than B). The implied thing being compared here is the experience of "working with you like this", and what they are saying is "of all the things I have known, none of them have been as fulfilling as working with you like this"
So I believe it is more or less your first sense. They are saying that "working with you" is more fulfilling than anything else (they have known).
If you remove the "as", then it becomes "I have never known anything quite fulfilling", which I think has more of your second sense. It is no longer a comparison of one thing to another, it is just a statement of "Nothing (I have known) has been fulfilling".
(in both cases, the use of "quite" is really just for emphasis, though it does imply that there might have been some things that came close, but they didn't quite make it)