Hypothetical case. You are.moving to a tropical island without internet but with an active RPG community. You have to choose 5 ars màgica supplements and ditch the rest because there is no space in your luggage. You can also keep one core book (any edition).
What would be your choices?
Realms of power the infernal. The best realm book IMO. Lots of flavor, lots of antagonists, and some cool magic systems that can we tweaked to represent other traditions as well.
Hedge Magic. For me it would be a putty having to choose between this one rival magic and ancient.magic, but I would choose this one since I think the traditions represent a much truer medieval magic approach than the core book. Along with spirit masters from ROP:I they cover most of what I like regarding job magic systems.
Lion of the North. The Loch Leglean tribunal book. My fav of all time. The core of the best foundation story in the line ever and a powerful nemesis for any saga. A challenging and evocative setting full of exploration, settled English lowlands and a clash of approaches to magic. An amazing campaign setting with a frontier feel both magically and mundanely.
Heirs to Merlin or Guardians of the Forests. The former because it is the vanilla settling, what most tribunals should look like. I do not like the fact that most tribunal need to be so distinctive as to be totally dysfunctional none with the other. GotF for me still kept the setting of previous editions true while having some local quirks, and it is rich and well designed with plenty of story ideas.
I would have also liked to keep triamore, houses of hermes, and some of the adventure books and hooks (including a.medieval tapestry), but alas, 5 books I said.
For the core book I would take the 5th edition one, even if it lacks the all important setting chapter on the orde of Hermes.
So, what would be your choices?