I'd have to say Pokemon Leafgreen/Firered. They're the remakes of the original Pokemon games, and adding new features such as the V.S. Seeker (Which was a big help when you needed to train your Pokemon without EXP. Share and the Elite Four) just helped make the game even more better than the first. With the release of these two games, a new form of connectivity in Pokemon games was introduced- the Wireless Adapter. Which means the introduction of the Union Room as well. (Although the Union Room didn't live up to it's reputation, for you could only connect with people sitting right next to you, which was a huge disappointment.. but it was still a decent feature and nice to use, better battling and trading with others via Link Cable.) Leafgreen/Firered also allowed you to explore new areas- the Sevii Islands. Each island had its own unique features- One Island had Mt. Ember (The new home of Moltres) and Treasure Beach (A fun place to scavenge around for items). Two Island had the Move Tutor (One of the handiest things in the game, you can teach previously-learned moves to Pokemon for just two Tinymushrooms or one larger mushroom, which comes in handy when you want to teach a move to a Pokemon that you previously deleted earlier) plus it had a Minigame Room. (A place where you and your friends can hook up together with the Wireless Adapter to play games like that Pokemon jumprope game and Dodrio Berry Picking.) Three Island was the one that had the remarkable Berry Forest. (In this vast area filled with trees, you could scavenge the entire area in search for berries. Everytime you visit, there will be new berries for you to find. A fun and smart way to look for these berries is to use the Itemfinder, which will point you out the direction of the berries you are looking for.) Four Island was the home to Lorelei (Which inside, included many cute Pokedolls, and you could make more appear every time you beat the Elite Four 25 times more) and hosted the Pokemon Daycare (Which was a bit helpful in that it raised your Pokemon for you at a small meager cost, but even more helpful in that you could breed two Pokemon there to recieve a Pokemon egg to hatch into yet another Pokemon.) Five Island included the Rocket Hideout (Where you had to battle the Rockets to make them scram), a tiny island (Where an old man was waiting to give you a Pokemon egg which hatched into a baby Togepi), a cave (Filled with a puzzle to solve and items for a reward inside), and a Rock Altar (Where a trainer is paying his respects to his deceased Onix). Six Island had a vast forest (Filled to the brim with many Pokemon, including the rare Heracross) plus some ruins where you could find the Sapphire to give to Celio, but some scientist takes it, so you must follow him to retrieve it). Seven Island was the one which included the Tanoby Ruins (Which will be filled with various Unown only when you solve the puzzle in another area close by) and the Battle Tower (The Place to compete for rare prizes, including the Upgrade, King's Rock, Dragon Scale, Metal Coat, etc.) Leafgreen/Firered also offered a great amount of chances to fill your Pokedex because it had the Safari Zone (Which is a huge mass of land filled with many rare Pokemon) and also had the one of the best Elite Fours in all of the Pokemon games (Because after you beat them the first time, they immediately became stronger for you to challenge them again and gain even more EXP fighting). After you beat the Elite Four and became the new Champion, the player was given yet another chance to fill his/her Pokedex, because the three Legendary Dogs (Raikou, Entei, and Suicune) were released into Kanto for the player to catch. All of these features are what make Leafgreen/Firered the great games they are today. So that's why Leafgreen/Firered really are great games. And I'm not saying that any of the other Pokemon games are absolutely inferior to these ones, I'm just saying that Leafgren/Firered made it pretty darn close to perfection. The only flaw was that they didn't make much of a long story plot, so they could have made it a much finer game by adding more mossions and tasks to complete.