Pokémon Red - the game that started what would go on to be the international titan of multimedia success that it is today. At its core is engaging, (mostly) well-balanced role-playing game with a consistent learning curve to progressively intricate obstacles to overcome. Combined with the challenging goal of completing the PokéDex, the game had incredible depth for its time.
[Matching PKMN Red & GameBoy Pocket. Also, just because I had Red doesn't mean I prefer Charizard over Blastoise. But I do love snagging an Electabuzz over Magmar.]
The multi-player functionality features both competitive (battling) and cooperative (PokéDex) modes. It could be argued that another version (Blue Version) is a gimmick; however, the version exclusives and trade-evolutions made interacting with other players essential to the experience. Furthermore, the game was perfect for the handheld GameBoy which allowed for grinding anytime, anywhere and allowed easier interaction with other players in the days before non-PC online multi-player.
Although I was young when I first played it, I have replayed it countless times throughout the years, (facilitated by the well-made remake of Pokémon Fire Red). This is due to the solid foundation of RPG mechanics, expansive and interactive environments to explore, and a dedication to the suspension of disbelief in a mysterious fantastical world. The game is not without some rough edges, many of which are refined in the many sequels that have followed (esp. the Special Defense stat and weaknesses to Psychic type).
This game is truly a masterpiece and should be given a try by both hardcore and casual gamers alike.
[A game so amazing that I've got ink of my favorite little fella - credit Dominique Darko Dominique Darko.]