Probably Charizard, since this entire gen has been GF trying to see how long until their jaws fall off. --- "Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod
These, and I'll argue that Clefairy could be included as well for its popularity and being Jigglypuff's rival. I read that the original plan was for Red/Ash to start with Clefairy and that Clefairy would be the main mascot of the series, but it was nixed and changed to Pikachu to appeal more to the boys they expected to buy the games. Plus, there was one popular manga where Red started with a rather block-headed Clefairy.
can you imagine how broken that would have been? starting with a clefairy?
EDIT omg i just had a realization. that might be why brock auto cures sleep status asap in rby. --- "when I'm in the process of doing something, I do it not because I think it's worth doing, but because I'm trying to MAKE it worth doing."