Thank goodness I live in the UK where almost every course is available for visitor play, even if there may be restrictions. Some are even available at weekends.
But as you mention Holy Grails, should we nominate Temple, Quaker Ridge, Vicar's Cross, Bishop Auckland, St George's, St Enodoc and every other saint from St Nom to San Cugat, not to mention pilgrimages to Rome, Canterbury, York, Lincoln....(don't think there's a course at Santiago de Compostella). And then there are those saints unique to Wales such as St Mellon and St Deiniol.... Plenty of scope for a very long list.