Mass Effect
To me, Mass Effect was supposed to be the next KotOR - and I fully expected an evolution of that excellent formula. The only reason I had to be sceptical was Jade Empire - which I wrote down to being a console-oriented experiment by Bioware.
So, I still had complete faith in Bioware, based on my adoration for their past games, most prominently Baldur's Gate, Neverwinter Nights and KotOR.
What I got was a game that was hugely inferior to KotOR in game mechanics terms, but also a game that was very powerful when it came to presentation.
My favorite part of Mass Effect is the sense of mystery and the powerful lore behind that universe. Listening to the Codex entries always had me glued to the screen (and speakers) - and I was fully engaged in that way.
Even something as simple as the introduction took hold of me completely, because of the music and the initial words serving as a presentation.
As a science fiction action adventure, I find Mass Effect to be nearly unsurpassed.
But, as I said, the mechanics were very underwhelming - and I still think KotOR represents the peak of Bioware.
As weak as the mechanics were, Mass Effect 2 was a significant step down - as Bioware thought it was better to eliminate the badly implemented features than actually improve upon them.
Beyond that, the story turned from intriguing mystery to "dark and edgy" TV show writing - and by the time Mass Effect 3 came around, everything good about the franchise was lost to me.