StarCraft!
I'd just gotten my first PC back in 1998 and StarCraft was the first full game I played. I was pretty bad (Keyboard shortcuts - what's that? APM over 20 - haha, yeah, no!) but still managed to finish it and Brood War, then all the custom campaigns, then I played all the multiplayer maps vs the AI, first Free for All, then Melee (all vs me), then I played them all again, multiple times, but with cheats. What cheats? "Black sheep wall" for showing the entire map and "Staying alive" for the game to keep playing after I'm eliminated. Then I'd pick Terran, set Free for All, have my SCVs kill the command center and then each other and then I'd watch the AI players duke it out. So yeah, I was spectating games before Twitch.tv and before it was cool!
Also Heroes of Might and Magic 3. Several years ago I'd estimated my total play time since I started playing it in 1999 and it came to at least 3000 hours, maybe more. My first PC was a Pentium 200 MMX with 32MB of RAM and when I played one of the largest maps in melee mode, it would take literally 1-3 minutes per turn to complete the AI turn. Good times, good times.