Arvo Part... Where have you been my whole life?

In pricipio is a newer release that I really like it has the song mein weig which is one of my all time favorites. I also love frates from tabula rasa as well as speigel I'm speigel. There are a lot of modern classical composers I like though they might not all be similar to arvo part they hit hard for me the same way he does. Scelsi, Nono, Xenakis, Penderecki are all favs and my above all fav is Gyorgi Ligeti.

I'll second, In Principio. Your list are certainly at the top of my own that I would have otherwise recommended. I'd add, Arbos, and Orient Occident. There is a compilation that you might also enjoy that includes Part as one of the artists. It is on Nonesuch (I think it was also on ECM for a while) and called, Silencio.

Another composer you might like, going a bit further in the realm of minimalist modern... You might try John Adams, On the Transmigration of Souls

More conventional and yet I might also recommend in association with the Part you mention... Henryk Gorecki: Symphony 3 "Sorrowful Songs"

Yikes! Been loving Part for eons, it seems. And he found the right label in ECM, as he gets sound quality to match his compositions.

If you like Part, you'll also like Giya Kancheli, another ECM artist. Same qualities and compositional sense. Doesn't use silence quite as well as Part, but a remarkable composer. As mentioned by Ejlif, Ligeti will meet those needs. It's also worth trying works by a Russian woman, Sofia Gubaidalina. Not quite as austere as Part, but in the same sonic family.

The Gorecki No. 3 is nice, but be careful. The most popular one, featuring Dawn Upshaw, is unmitigated crap. Almost ANY of the others in catalog are better. My absolute favorite, if you can find it, is on Olympia. The soprano is Stefania Woytowicz.

Returning to Part, an overlooked work of his is "Kanon Pokajanen." Gorgeous vocal piece.

A bit different from those already mentioned but definitely worth checking into is Valentin Silvestrov.

I found that the only solution was to buy them all. And then listen to them in order. I found that quite interesting - so much so that I have done it a few times. I have my favorites but it is an interesting 'journey'. I enjoyed that so much that I started doing it with a variety of composers.

There are some great "complete works" boxed sets out there for not huge money. Going through them in chronological order (where possible) provides insight into how the composer's music changed over time (assuming that you trust the interpretation - and that is, in itself, an issue one has to deal with as well). Doing so for a few composers is a great education!

But back to Part. I like Missa Sillabica, Magnificat, Berliner Messe, and possibly my favorite, Miserere. The most accessible of his music might be "I am the True Vine" (if you don't like that one, you might as well skip the rest - if you do, then there is a world beneath which is wonderful to dig into.

Arvo Part is kind of off by himself in terms of style, but I find Tavener (for obvious reasons) and Britten (for reasons I am not sure I can clearly explain) have similarities. I also think fans of Arvo Part who have not listened to Ernest Bloch might enjoy it. Ditto for Bartok, though one might have to listen to a fair bit of him to see the links - but don't worry, it's definitely worth the journey too!