Gold, Silver, Crystal
#152 (Chikorita) - #251 (Celebi)
Falkner, Bugsy, Whitney, Morty, Chuck, Jasmine, Pryce, Clair
Elite 4 & Champion:
Will, Koga, Bruno, Karen, Lance
Entei, Raikou, Suicune, Ho-oh, Lugia, Celebi
Generation II was arguably the real basis for the series as it stands today. Gold Version and Silver Version introduced many key features that are now staples of the series, and they even modified some aspects previously developed in Generation I. On the off chance any you have not played the originals, you may be familiar with much of the content due to the recent remakes HeartGold and SoulSilver. We'll come back to them in Generation IV.
Of course, the biggest addition to Generation II was 100 brand new Pokémon. These were added to the original 151, bringing the combined total available in Generation II to 251. Not all of the original 151 were available this time around, so trading with friends or bringing your old Pokémon from Generation I into Gold or Silver was necessary to complete your Pokédex. Most notable were pre-evolutionary forms of many Generation I Pokémon such as Pichu, baby form of Pikachu; Magby, from Magmar; and Cleffa, from Clefairy, amongst others.
So how do you get baby Pokémon, you ask? By using a new mechanic that is now one of the most popular aspects of the game and vital in the competitive metagame: breeding. In Generation II, Pokémon were first given genders and, when you reached Goldenrod City, you could stop by the Daycare Centre to breed them. Implementing breeding allowed players to effectively duplicate certain Pokémon while increasing their stats and potential. Legendaries were unable to breed, but almost all other Pokémon were fair game.
After your two Pokémon mated, you collected an egg which hatched after a certain number of steps. The new baby Pokémon would be the same species as its mother (making for some interesting scientific discussions amongst Pokémon professors, I'm sure), but could also inherit the moves of its father, even those taught by technical machines. Inherited from both were the hidden IVs (Individual Values), an incredibly complicated and in-depth topic. In essence, with good IVs it was possible to boost the base stats of your Pokémon even further. Suddenly, whole new opportunities were opened for creating your ideal and unique Pokémon team.
With the new Johto region to explore, there were eight new gyms and a new Elite 4 to conquer. Even Red, the hero of Generation I, was available to battle on Mt. Silver after beating the main game. To this day, he has the highest level Pokémon of any NPC trainer in the whole franchise. Taking down his tremendously powerful team, capped off by a level 81 Pikachu, was a truly momentous feat.
On the battle front, 86 new moves were added and new TMs were introduced. Surprisingly, Generation II even brought two new Pokémon types (the only generation to do so): dark and steel. Many Pokémon were given a secondary type which added a new layer of depth to determining the resistances or "super-effective" weaknesses of particular Pokémon.
Outside combat, major changes were introduced to the world. Generation II was the first to bring berry trees, the ability for Pokémon to hold items, a day and night system, updated colour graphics and new types of Pokeballs. Last, but not least, are two game-changing additions: a playable female character (in Crystal Version), and the introduction of mind-blowingly rare 'shiny' Pokémon. These Pokémon were colored differently to their regular counterparts and your odds of encountering one were less than 1 in 8000. Shinies have become a popular part of the series and are still highly sought after and collected today (I've only ever caught one...).
The only Pokémon game I played was Crystal, but I found it highly addicting. Even with bombastic 3D polygon graphics coming into their own in recent years, the colorful sprite and tile world immersed me to the point where I wasn't just playing Pokémon, but living the life of a trainer. What I liked best about Crystal was being able to choose the gender of my protagonist. If given a choice, I always play RPGs as the girl, so I was happy to do so in Pokémon Crystal.
A hundred new Pokémon? They had to be kidding. But no, they weren't, and now I had 100 more Pokémon stats and numbers and appearances to memorize and catalog in my mind's Pokédex. And if they weren't all as adorable and cool as the original 151, the sheer number of them made up for it.