I don't have a ton of their offerings to comment on, compared to some here, but I know that Albion V, LCO Strings, and the Eric Whitacre Choir are masterworks.
Alternative Solo Strings is perfect at what it does, and in a way, you can use it as a more playable version of some of their EVO-based strings.
The Studio series must be one of the best starter options out there. It's far from perfect, but what it enables you to do at its price point is pretty much unrivaled. The Bernard Herrmann Toolkit is a bit niche but still versatile, and fills in some gaps (percussion etc.) in the Studio line, locking in perfectly with it (same room, players (I think), microphones, engineer).
Like others have said, the LABS stuff is incredible. I have quite a few of the old Kontakt Labs instruments that have been repackaged in the Spitfire Player, and the new ones they've been releasing are really impressive too.
The only Spitfire library that I own which I think I would resell if I could is Orchestral Swarm, but that's my fault for making a somewhat hasty purchase, and not being able to find much use for it in my music. It's a very cool set of sounds.