You should say the greatest worthy rival to Celtic, and there is club team such as Rangers. I am the same as Celtics, and Scottish Glasgow is a stronghold. Celtic is club team receiving Scotish (Gael) majority support of a progeny of a Celt in Catholic system. In Ireland, Northern Ireland, it is popular probably because I do it with the club which had the same culture background.
F.C. such as Rangers is classified in Anglo-= Saxon system in vice versa, and there are many federationists (a unionist) who are a sympathizer of the U.K. alliance of a national church group English for a supporter.
There is a considerably complicated background so that religious relations of politics of a race are included as above about Celtic and Rangers. Therefore there is the problem that it is difficult to tell it simply only at the side of sports. Relations of such a rival are called "the old farm Derby".
As for Celtic and Rangers and Glasgow Derby, there is reputation of the most dangerous Derby all over the world. I become the greatest event to be excited with both athletic supporters very much from old days. I heated up so as to have sent a dead person in the past, but in late years calmed down to some extent. But medical care is necessary even if I put it after a match at present.
It is different from Japan that there is a rival of two teams in one town such as Glasgow. I am wearing a uniform after a match was over and walk the city because one's favorite team won and must avoid that you enter a shop so that there is danger to rub feelings of a partner team up the wrong way.
In the world of sports, there is a rival in any kind of team, but after all Rangers can be said to be the best worthy rival for Celtic.