I was blown away by Mass Effect because of the seeming realism of the universe.
There was a lot of attention to detail in terms of the history, races, technology. It made the plot that much more engaging.
That wonder was never really replicated in Mass Effect 2 and 3, simply because the ground had already been broken.
They were obviously better games, but I'm not sure if they were better gaming experiences for me.
But that's typically the way it goes for the first of any ground-breaking series, whether it be a book, movie or game.
I was actually behind the curve on Mass Effect - I wasn't all that interested in a new intellectual property after the Baldur's Gate games and KOTOR, but it was Bioware so I gave it a whirl.
They followed up with Mass Effect 2 and Dragon Age : Origins back to back (in the same year?) and it probably represented the zenith of Bioware IMO.