continuous derail!I and all my friends would easily side with ME1 Saren/Sovereign plot over the ME2 collector/Cerberus plot.
With that said ME2 stepped it up in many ways so it's hard for me to pick between the two. ME1 gives you that mysterious universe something is out there feeling while Me2 makes you feel more important.
Both placed in my top 10 games of all time if I had too.
I enjoyed ME1 better because it's open world and is a bit of a scifi mystery. Plus you get to explore the Mass Effect universe the first time and all the details they present were excellent. I wouldn't put the Saren plot as being that much better or worse than the collector stuff.
ME2 has better combat and some of the most interesting team-mates of any squad-RPG ever.