That's a difficult one...
I think my preference is for Ghost In The Machine, because it has my favourite Police song on it (Every Little Thing She Does Is Magic). Invisible Sun is another masterpiece on that record.
On the other hand, Synchronicity has Tea In The Sahara.
Then again, I'm not really a fan of the songs Synchronicity II and Mother, with the latter being fun but disposable.
I have that one. Bought it really cheap a couple of years ago. It's also the only Police release I have. But then again, if you have this one you have almost everything they ever released.