Both are Ok! Instead of "in" it is better to say "about". a) It is the only thing that people find interesting about me. b) It is the only thing about me that people find interesting.

Answer A) sounds more fluent.

Thanks a million for the detailed explanation ^_^

Both are correct and mean essentially the same thing. The very subtle difference where the greater emphasis is placed. It is the only thing that people find interesting in me. - To a native speaker this sentence is placing a greater emphasis on the subject "me". It is the only thing in me that people find interesting. - This sentence places a greater emphasis on the subject "it". The difference is minimal so you won't confuse anyone by using either.