That’s a broad question, since each Church has it’s own winner.
I guess among the Byzantines in the US, the honor goes to the Ukrainians, where it seems to be St Michael’s Parish in Shenandoah PA , established in 1884. The Ruthenians weren’t far behind with St Mary’s in Freeland PA, founded in 1886.
For the Maronites in the US, Our Lady of the Cedars in Boston, dating from 1890, is usually cited since it’s the oldest continuously functioning one. St Jospeh’s in New York was established the same year, but sadly it no longer exists. (It is said that there was a congregation in New York some years earlier, but for reasons that I don’t recall offhand – perhaps the priest passed away? – it went into abeyance until St Joseph’s was established.) I could be wrong about this, but for the Maronites in Canada, I think the honor goes to St Peter’s in Windsor ON, dating from 1924. (I believe there was a parish in Sydney NS from 1909 – also St Joseph’s – but it closed in the early '50s. The parish was reincarnated in Halifax as Our Lady of Lebanon some years later.)
I know the above isn’t much of anything, but it’s a start.
