Checking/Ticking all the fields on a recommendation letter as "Exceptional" My student has given me a recommendation letter form for the studies he is applying for, and the form has a part where I should simply "tick" the options. For example: Maturity, Motivation, etc. I can check these fields with "Exceptional", "Excellent", "Very Good", "Good", and so on...

When I think of it, I really want to tick all these fields as "Exceptional", because this student is my best student, he has been the top student in his class for all the courses I have taught and he is one of the best students I have known throughout the, at least, past 10 years. Do you believe that checking ALL the fields as "Exceptional" would look very bad to the admissions committee? I do not want to ruin his chances by my recommendation letter.
There is nothing wrong with checking "exceptional" for every question.  Working on graduate admissions, this is not uncommon.  Of course, be sure that the student really is exceptional in every area.  While he may be brilliant, is he really exceptional at writing skills, oral expression, self-confidence, etc.?  I actually have a more positive response if a trait has a slightly lower rating, so I know that thought has gone into the ratings, although I don't recommend doing this if the candidate truly is exceptional in every way.

If the recommendation form truly does not allow you to attach a letter, use the space to say (as you do above) that the student is one of the # best you have worked with in the past 10 years.  If you have more space, comparisons to other students are helpful, for example, "X is one of the top 3 students I have encountered in the past ten years. The other two went on to top PhD programs, where they have both flourished.  The one who has graduated is doing a post-doc at Y, and the other is making good progress to her degree."

You can also send a letter even if one is not requested.  I don't see how it could hurt (unless you had something negative to say).