This topic has been covered before, and several times - a quick Search turns up this thread: https://forum.atlas-games.com/t/answering-which-books-should-i-buy/2382/1 , which is as good as any.
In short, rough consensus (in no particular order?) seems to be...
HoH:___ that covers whatever House your mage belongs to. (As a SG, choose the one that applies to most of your NPC's - often HoH:True Lineages seems to win out, covering Bonisagus, Guernicus, Mercere, and Tremere - those 1st three are just so central to many sagas, and the additional insight into The Code, original research and the RedCap mindset is invaluable.) If you're lucky, one might cover the majority of your Players. A SG should own all 3, but if the Troupe can cover them all, that's good enough.
RoP:___ that covers whatever Realm is a "theme" for your Saga, or of special interest to your mage. (Note - make sure your SG is as excited about it as you are - if you buy RoP:Divine, that doesn't mean your SG will get excited about inserting heavenly plot elements. Same with Infernal and Fae. RoP:Magic is the best bet of the 4 if you have to bet blind.)
Covenants is not critical, but rich color for any saga. Labs, texts, expanded hooks and boons, covenfolk, Rego craft magic - you won't regret it.
___ Tribunal that covers your Saga. Harder to make use of the Levant if you're all in Scotland.
Beyond that, it's personal interest and Saga focus. No snoozers in the bunch if the title sounds of interest to you.