Simple Question. For me it's either Encore or Curtain Call. Believe it or not it was When I'm gone that really got me into Em. I bought Encore in late '05 and loved it (wtf I know right) and a few weeks later bought Curtain Call which really made me want to buy all his albums.
I really have a soft spot for Encore just because of its nostalgic value. Reminds me of good old times playing the PS2