Mass Effect was released just a little too late to be included in my all-time top 100, where it would have secured a high spot. ME2 improves on the original in almost every way.
For starters the overhauled engine is so much better you wouldn't believe the two were related. On PC it's much less of an issue, but the difference between ME1 and 2 on Xbox 360 is night and day. It's not just visual though... in every facet of the sequel Bioware stepped up and delivered the goods. In some quarters the streamlining of the interface was regarded as dumbing down, moving the game closer to an action adventure than an RPG. In reality it removes a lot of cumbersome and frankly unenjoyable busywork and lets the player focus on the thrilling story and enormously improved combat.
Mass Effect as a whole is totally irresistable to a space opera fan, and through both games it really feels like you're an integral part of this complex, expanding story. In true Bioware tradition there are tough choices to be made that significantly impact the subsequent path of the story, and beyond that there are dozens of callbacks to minor incidents from the first game (importing a character from ME1 is an absolute must). All this of course adds huge replay value. The choice to put instant Paragon and Renegade actions on a mouse click without ever telling you exactly what will happen is a brilliant move as well, with often surprising and hilarious results.
There's some astonishing performance work going on too. While there are always moments of iffy animation they are completely overshadowed by the quality of movement and expression in cutscenes. The writing and characterizations are also uniformly excellent (in particular the Asari character Samara blows my mind with her realism and performance), and the extent to which Bioware has gone regarding relevant dialogue depending on your characters is very impressive. It's a game with really no poor dramatic qualities to speak of. Structure-wise, where ME1 was an epic quest with a strong story thread, ME2 is more a sequence of short adventures with an overarching main plot - different but no less accomplished.
I seem to be alone in lamenting the loss of the planetary exploration aspect, which to me was a big part of feeling like I was truly 'out there' in the unknown reaches of the galaxy. I also seem to be alone in enjoying the planet scanning and mining aspect that they added in its stead (though even I admit that's not quite so addictive on multiple playthroughs).
Bioware have fashioned a compelling universe to play around in. It may borrow heavily from established sources but it has enough of a unique identity to stand on its own. Where a lot of game series slog through unwelcome sequels of decreasing value, here the proposed trilogy doesn't feel like enough. I really didn't think anything would beat Mass Effect 2 for me this year...