I'd say another Jynx, but the Cave of Ice banned episode techincally did that, so yeah.
Porygon2, no question.
Magmar. Beyond Blaine's Magmar, we have no other episodes featuring the sunspoter here. For those who wish to comment on Satchel's on "You're a Star, Larvitar (I think the 5th season finale), that was mostly on Magby, the pre-evolved form.
Magmar is quite the powerhouse and rather viable, should have some more prominence.
Flaffy, they ignored it throughout Johto and finally brought it out in Hoenn. Just more a "whoops, we forgot." if anything.
Furret. It never had a real episode per se. Just a Pokemon who was more there if anything.
I'll keep thinking of this.