Diablo 2 in HC over dial-up in South France was fun, especially with the random disconnection that was plaguing different providers...
Duke nukem 3d. Dialup. Wow thats pure nostalgia!
It would go out of sync and switch to single player.
Doom was ok over dial up computer to computer. Quake was even slow connected direct over modem. I would guess the slowness was the extra data for the 3rd dimension going over the wire.