Age of Empires 1 demo.
I had meant to get the AoE2 demo, but we had AoE 1 instead. I was always into history more than my brother, he was playing while I watched in sheer awe (I was like 7). My dad watched too, and pretended to be annoyed when the baddies got killed and had blood spill out from them--and he made a few remarks about the enemy houses burning down with little flames poking above them.
For a kid who only had played Sim Tower, Sim City, and Sim Town, this was a colossal shift in understanding what was possible in a game (I had always thought that games were either those dumb fight club games, horror shooters, or Sim City lol) and it definitely changed my life.
We bought AoE 2, but I liked the demo for AoE 1 so much I went back and bought AoE 1 as well, as well as the Rise of Rome expack.
More recently, the Battlefield 2 demo definitely sucked up my life. The demo is the third most played game on my Xfire account, stacking up an impressive 185 hours, but I had been playing the demo since June of 2005 and didn't get xfire til around March 2006--so you can guess this number is a little off.
Battlefield 2 itself is 420 hours.