Mass Effect: Mass Effect 2. I LOVE Bioware's sci-fi RPG saga. Utterly amazing universe they've created, awesome characters, story, dialogue, voice work, visuals, presentation and so on. Mass Effect 2 refined a lot of elements from the first game, was much more in-depth with the stories of your companions, and that final mission, great stuff!
Baldur's Gate. BG II: Shadows of Amn. Another Bioware series. I LOVED them all. Taking my awesome Mage from the first game into the expansion, into the sequel, and into it's expansion and finale was just incredible. Shadows of Amn was just a stunning sequel, better resolution, so even better visuals, especially the backgrounds, great story and villain, great companions and conversations, higher level stuff, TONS of content, and content that's all enjoyable to go through. How a sequel should be done. More! Better!