Ursa, the Lady of War leader of the Proud Warrior Race the Barbic Gummis, judging by how many fanfics feature her as a character despite her appearing in only 4 episodes, none of which have been released on DVD.
The Knights of Gummadoon, specifically Sir Plucki, Sir Gumlittle and Sir Blastus, only show up in one episode and yet appear in tons of fanfiction and fan art. Quite a bit of fanfiction finds ways to bring them back into the main cast.
Chummi Gummi, who shows up once in "Up, Up and Away" is a fan favorite.
Germans Love David Hasselhoff: Czechs love, nay, absolutely adore Disney's Adventures of the Gummi Bears, and they have taken their dedication to amazing levels.