In regard to Tenten:
Gohn's always preferred fighters that don't focus on 'magic-esque' abilities, e.g. Tenten and [somewhat] Rock Lee.
Now, Tenten has an interesting and unique fighting style, and Gohn might even go as far as to say that her style is his favorite, but anybody that declares her as having "untapped potential" is moronic. She has never accomplished anything in the entire story, aside from getting curb-stomped by Temari; she also lacks any overpowered bullshit that'll help her compete with the primary characters.
EDIT: Wait, how and when did Luffy lose to Don Krieg?