It still breaks my heart that Woolio was not reintroduced in the Welcome amiibo update, even though a lot of the other Gamecube-exclusive villagers (Sprocket, Annalise, Dobie, etc.) were. He was one of my first villagers and still is one of my all-time favourites
Iggy the old jock goat. I'm not sure if anyone else likes him, but he was(and might still be) in my GCN town, and after e+ he never came back.
Also, Champ. He has disappeared ever since Porter returned, and he was the only villager who was in Wild World and City Folk who didn't return in New Leaf.
There are only three for me since the Welcome Amiibo update: Betty, Belle, and Champ. Betty and Belle were among my first six villagers in my GC town. Also Champ because he's the only monkey villager in the game (not counting the gorillas of course) that I actually like.