By “Westernmost” I mean which Eastern Catholic Church is most like the the Roman or Latin Catholic Church?
Which Eastern Catholic Church is the most different?
given the environmental and ritual approaches, the Maronite Quorbono is most similar to the Roman Mass of the Ordinary Form. Ad populem, three readings, variable consecrations, instrumental music, no iconostas.
But, by the same token, the Maronite Quorbono is of Syriac derivation, and is a mixture of parallel development and later latinization. It’s been lamented that latinization is so prevalent and ancient that removal of much of it would be a breach of tradition.
The Extraordinary Form? harder to pick, but again, the Maronites hold it, and the Armenians and Chaldeans are definitely in the running.
Theology? much harder to say… but quite likely to be the Coptic Church Sui Iuris, but I’m not certain. The Byzantine Churches are recovering their theology, most of which is encapsulated deeply within the Liturgical Propers.
Administratively, the understanding of the role of Bishops, Archbishops, and Patriarchs is the Coptic church, from what I’ve seen. Byzantines (8 of the Eastern churches!) seem to put far more power in the hands of the pastor than the Romans. Most of the Eastern Churches are also far more council-driven and concensus-seeking than the Roman hierarchy. Like the Romans, the Coptics (Oriental Orthodox and Catholic both) are noted for a standard of authoritarianism on par with Roman.
It varies highly on how one defines “alike” and what parameters one is looking at.
For Praxis, the Maronites. For Doxis, probably the Copts, but could be others.
But each is a unique flower from the Church of Sts. Peter, Paul, James, and Andrew, each growing in its own way, sometimes towards one another, sometimes in differing directions.