I think that the most popular item among the ones designed by players, which is not a fansite item is well known Nightmare Doll!
It was created by Li Dao, one of precursors of Tibian art.
the only item chose by Cipsoft after the old volunteer era, without winning the 1st place in a fansite contest (actually he got the 2nd place with it on TibiaFriends contest).
My source was Tibia Royal but the fansite no longer exists so the link doesn't work~