Sox current rivals are the other two teams that seem to be good at the same time in the AL Central. Twins and Cleveland now, a few years ago it was KC and Detroit. Its more cyclical. Division play is so heavily weighted in the schedule and the AL Central has been around long enough that other older rivalries have faded.
The one team I do not consider a rival are the Cubs. I hate them with a passion but those 5-6 games each season matter little to the overall standings. Fanbase rivalries aren't the same thing.