First something about tournament categories : there is a classification, based on the average ELO-rating of the competitors. Each category has a span of 25 points, and it starts at 2250.
E.g. cat. 1 is 2251-2275, cat. 7 is 2401-2425, cat. 16 is 2626-2650.
Some examples : Linares 1993, cat. 18 ; AVRO 1938 cat. 17 ; St Petersburg Final 1914 cat. 16 ; St Petersburg 1895/1896 cat.15.
Let somebody else add category 18 and higher of the last 20-25 years, please !
Which, in your opinion, is the strongest chess tournament ever held?