Netreck, circa 1989. Turned into an obsession that lasted my entire college career, to the point where I had actually written my own custom borg client. To date, I still haven't found a team based multiplayer game that's hooked me the way Netrek did. Capture the flag doesn't even compare to the variety of team strategies that Netrek had to offer. If anyone makes a modern update/clone of it, I'm totally fucked.