Tales of Mythic Europe
Magi of Hermes
Lords of Men
But then, I'm biased.
Okay, some questions:
Do you do much storyguiding? If not, is that something you'd like to do? (I think you've answered the first part, but bear with me)
Do you have a hankering to play characters based in any of the four realms?
Is your magus exploring mysteries or exotic magic?
How important are companion and grog characters in your saga, and are you looking to invest more in them?
And now the reasoning:
The House and Tribunal books are great sources of stories for your fellow players. You'll always find something that you can tailor to them. For instance, if one of your fellow players has a Bonisagus, True Lineages is a gift of little stories and events that you can adapt. And the Tribunal books can inspire relationships between your own saga's magi and covenants, which again gives you content.
The Realm books tend to speak for themselves, but really if you want to play a faerie or a diabolist or someone gifted with Divine power, your best bet is to pick up the appropriate book. It's fair to say that the Realm books really hit their stride later on, but I have been finding Infernal indispensable of late and the Divine is still really useful even in Mundane situations (they all gotta pray, right?).
Mysteries and Ancient Magic are heavily used in my saga. Our whole covenant is built around the principle of rediscovering and protecting and old mystery cult, and my Verditius is seeking out inspiration in bending Heron's arts to his own magic. It means that all of our magi are doing something very different all the time.
Art & Academe and City & Guild will really sort out your companions and grogs. Once my current companion character dies of old age I'm definitely looking to bring in a physician based on the A&A book. And C&G has been quite useful recently too.
So in rounding out your collection, I'd advise looking at what you're most likely to use. They're all a damned good read. I mean, I've not even mentioned Hedge Magic Revised, which is superb if you just want to try something a little different. Or Covenants (which built my workshop out nicely, thank you), which is almost required reading.
So take a look at what you're most likely to use, or what really intrigues you, and buy in that order.