While the original Baldur's Gate provided a dizzying expanse of territory to explore, Baldur's Gate II added focus. The environments were still enormous, but there was direction, a powerful sense of motivation and never a lack of personality. Shadows of Amn is not only one of the biggest role-playing games, but one of the most dense and most detailed. It's an exhaustive example of obsessive, triumphant world-building, from the staggering amount of dialogue and character development to the fine touches on remote wilderness. The lynchpin of the entire experience was a fascinating villain who was as captivating as he was despicably dispassionate, giving the story momentum and keeping the tension high. It's one of the few games that combines a colossal world with equally deep gameplay, as the Dungeons & Dragons combat mechanics and six-person party allowed for an incredible amount of tactical depth with every encounter. Baldur's Gate II is the type of all-encompassing role-playing experience that may never be matched.
Ultima VII: The Black Gate
It's impossible to quantify the profound influence this game has wielded over subsequent games, and not just RPGs but any open-world game in which the player is allowed to explore and live in a bustling world. Creator Richard Garriott lauded it as the "most masterfully created" of his series. It was a work of that era when text and keyboards were being swept aside by mouse and slick graphical interfaces, when grand high-fantasy themes were being tested and subverted. Players could hang around in a pub or bake a loaf or bread and, although the plot circles around a murder investigation the game could be enjoyed without absolute adherence to the main plot-line.
Though the combat was built using deep and complex Dungeons & Dragons rules, brilliant character development and writing are really what fueled Planescape: Torment. It's easy to lose hours of play time engaged in absurdly detailed conversations about everything from the mundane to the nature of existence. Torment's hilariously twisted setting serves as fertile ground for all manner of wild stories of murder and alternate dimensions, and every character is developed to an extent that puts most other games to shame. Throughout it all the writing's wit never dulls, maintaining a blend of razor-edged humor from one captivating conversation to the next, developing a sense of personality and place more effectively than most of its role-playing peers could ever hope for. It also didn't hurt that the environment design was simply gorgeous.