Gold and Silver. Yes, I'm showing my age. {XD}
Although there were rumors going around, as well as hints in the anime (ie. Ho-Oh, Togepi, Marill, etc), the idea of 100 new Pokémon being added to the games seemed too good to be true (especially at a time when even Mew's existence was still shrouded in mystery). While new regions and Pokémon have become an expected (and predictable) feature of the games nowadays, back then, it was truly something new and unprecedented. The countdown to GS was fun and exciting.