I think that the reason "TREASURE ISLAND" always does so well is that the movie has many interesting locations and a very illusive villain. Also, it seems to me that many Chan fans are also intrigued with the inclusion of the "supernatural" and magic. Eve Cairo's psychic bits, Rhadini's magic, and the seance portions involving Dr. Zodiac fill that bill to overflowing in this movie.
Admittedly, I like this picture very much, although I chose "THE BLACK CAMEL" as number one. I think what appeals to me is the use, for much of the action, of the west coast World's Fair of 1939, held at San Francisco's Treasure Island. The whole thing just has an intangible "feel" to it for me that I find irresisible.
The fact that so many Chan films made it to the poll indicates, as you suggest, that the opinions of Charlie Chan fans are exremely varied!