Few random impressions:
I generally prefer discs devoted to a single composer, which may be a reason I don't buy so many ECM NS.
Agreed that the Kurtag stuff is good.
Re. Arditti, I recommend the Ruzicka, but happen to dislike Gavin Bryars's music (YMMV).
In Renaissance polyphony, I generally find the Hilliard Ensemble best avoided (better alternatives elsewhere).
Holliger is good, though I cut back because my appetite for 12-tone art songs is limited.
B.A. Zimmermann is a favorite of mine; the Bach/Zimmermann disc with Thomas Demenga is good.
The Lachenmann stuff is worth a listen, perhaps also the Scelsi (esp. the disc with Uitti) if you don't already own recordings.
ECM NS records a lot of Tuur; I liked a couple of his recordings (rock-influenced classical) but had limited appetite.
Ustvolskaya (for sure) and Gubaidulina may be worth investigating, subject to my comments on multi-composer CDs.
The disc "Voci" with Berio's Folk Songs would be good if you don't already have recordings of the Folk Songs (the non-ECM with Cathy Berberian is preferable IMO).