If you thought it couldn't get any better than R/B/Y, you thought wrong.
Like the first Pokemon games were pretty muich the reason most people bought a gameboy in the first place, Pokemon Gold and Silver were also way above the level of most other titles (except for both Zelda Oracle games). The catchy music, happy monsters and moralistic gym leaders returned and this time you were finally able to see Misty's hair was actually red. You embarked on your way to the top (Pokemon league Champ) in the village of New Bark Town in a land west to the familiar Kanto region. In this new land named Johto so many things were introduced (phone device, in-game time, day/night difference, radio, lottery, berries, hold items) and improved (pokedex, more moves and monsters) you would ask yourself how you enjoyed the older versions. They seemed pretty simplistic compared to Gold and Silver and the only value that game had left was pure nostalgia. And Game Freak knew. So the guys at Game Freak decided to leave Kanto in the game, allowing the player to visit the places he loved, earning 16 badges and even battling his avatar of a few years ago (Ash). The only reason this game isn't perfect is the radio and other features were pretty useless, and thus had little to no value but sounded pretty innovative. The day/night system was however a complete succes adn till this day it is not clear why for god's sake Game Freak removed this feature from the third generation games. Of course most of Pokemon's success lies in the fact the programming knows its flaws, like the mysterious MIssingno. creature really added to the fun of the previous games. In Gold and Silver the Missingno. is gone however a lot of cloning is going on, which is even more useful now pokemon can hold items. Also in Gold and Silver you can search for two legendary pokemon (instead of one, Mewtwo, in R/B/Y), and it depends on which game you're playing (Gold or Silver) which one you are to encounter first. Problem of course now no one watched the anime anymore people wouldn't get used to the new pokemon that had no nostalgic value whatsoever (except for Ho-Oh), but a few really great new monsters were created like Skarmory and Tyranitar. Talking about those two, I shouldn't forget about the two new Pokemon types that really helped making the game even more diverse (and weakening the Psychic type). Also the Special stat was now Special Attack and Special Defense, which was only a natural change and helped making Mewtwo a lot weaker too (Mewtwo had incredible Special in R/B/Y but weak Special Defense in Gold and Silver).
Graphically the game had brilliantly improved, buildings had better designs and Snorlax was now four times as large as you. In fact, Ruby and Sapphire didn't even add to much to the graphical beauty of Pokemon because Gold and Silver looked great already.
A 9.6 is not a 10. So what could've been done better? Two things: firstly the safari zone was left out (Missingno. problems?), second the HM Fly couldn't get you anywhere you wanted because your pokemon couldn't fly from one region to another. However the magnet train and S.S. Aqua were very welcome ways to travel between Johto and Kanto.
Pokemon Gold and Silver were the best sequels possible for the franchise, and even managed to improve on the already nearly perfect formula of R/B/Y. I deem this the best sequel ever, as well as the best handheld game ever. There's a reason I played it for over 800 hours.