Some of the scrapped/altered Pokemon from the Gold/Silver '97 Space World demo were pretty cool, and I'm a bit upset we never got to see them officially.
From the scrapped Pokemon from Gen 1, I like Crocky, Jaggu, Gyaon, and Deer.
From the scrapped Pokemon from Gen 2, I like Rinring, Bellboyant, Kotora, Raitora, Wolfman, Warwolf, Sunmola, Anchorage, Kurstraw, Pangshi, Bomseel, the Honoguma line, the beta Farfetch'd evo, the beta Qwilfish evo, and the beta Girafarig prevo.
Kotora and Raitora and its lost third evolution. Honestly, I'm so mad these Pokemon were cut not from generation one, but from generation two as well. Wtf.
Losing baby meowth and growlithe was a TRAVESTY!
I also really liked the original design for politoed and I wish some farfetch'd could evolve into Madame. Also the alternate weepinbell evo was cool if derpy