Where did Don Knuth say that suffix trees were the "Algorithm of the Year 1973?"
The source of the quote seems to be From Ukkonen to McCreight and Weiner: A Unifying View of Linear-Time Suffix Tree Construction by Giegerich and Kurtz.

On page 347, at the start of Section 6 (An Explanation of Weiner's Algorithm), they write


  In this section we go back to the roots
  and take a look at the “Algorithm of the Year 1973” (D. E. Knuth according to [19]).


Reference [19] is an unpublished manuscript:


  [19] V.R. Pratt. Improvements and Applications of the Weiner Repetition Finder. Unpublished manuscript,
  Cambridge, MA, 1973.


Apparently these are lecture notes (this is how they are referred to in the monograph Image and Text Compression by Storer).