'In' would be used specific(ally), and 'of' would be general(ly).
Your English snobbery is rearing its ugly head once again !!
Noblesse oblige, my friend.
If you refer to something that is the best in an professional area (no noun implied just 'the best') like say marketing, bodybuilding, design or even fishing.Do you say "the best of marketing", or "the best in marketing" I am inclined to use "in".
are you using "marketing" as a verb ( a word describing action/ actions ) or as a noun (a word describing an object )? objects) word
IN, you animal, that is not even a hard question