Since I was sort of a Sonic junkie back then, it was Sonic Adventure 2 for me.
It was 2004, after playing all of the old school classic Sonics on emulators, Sonic Adventure DX and Sonic Heroes on PC, Sonic Adventure 2 was the only major game at the time I haven't played yet. Since there wasn't a PC port of it back then, either the Gamecube or Dreamcast version was my only option. (I haven't even heard of the Dreamcast until around 2003 when we got broadband Internet, the last console I even knew about was the Super Nintendo since my brothers rented one a few times back in the day.) I tried DC emulation on PC, but that was still pretty buggy back then. When I read that you can burn DC games on CDs, I decided on that instead of the Gamecube version, purely to save on money, lol. (I was and still am a cheapskate.)