The first Mass Effect will always be my favorite. I didn't cop it when it came out, and only got around to renting it several months (maybe almost a year) after it came out. As soon as I started playing it, I was hooked. I had previously played KOTOR 2 and I liked the combat system, but the story to ME1 is so far ahead of that of ME2 and ME3.
I just felt like I WAS Commander Shepard when I was playing. The mystery and revelations, piece-by-piece. The "Hold The Line" speech at Virmire
. The dialogue with Sovereign is the creepiest/coolest gaming moment I can remember.
The decisions. Kaidan, my dude