Narrowing it down to just one disc is almost impossible and there are still so many discs I want to try out. A few thoughts, however:
The Leopard as many have mentioned is a great disc. A great fairway driver in many situations and it can be anhyzered with great effect as long as there's no cross wind or head wind. Definitely not a good headwind driver, and gets a bit understable after it gets beat in (at least for me).
My all time favourite disc is the Aero. I've been throwing one for over 20 years. Pretty old school, but it just goes where you throw it and holds whatever line you give it. It really doesn't change much as it wears in until eventually, years later, it gets Alzheimers and just starts wandering around at random. It's kind of like a roc with more glide and very little fade. That said I'll be picking up a Roc to add to the bag of tricks. I can drive the Aero over 300', use it for approaches, putt with it. A great all around disc. Not for a forehand though!
I also love another old school disc - the Eclipse. Very controllable, excellent S curves, good roller. Just don't anhyzer it high and hard and expect it to come back.
Of the big drivers, I like the Pro Wraith. My current Wraith is Star and I find it a bit too overstable for my arm. The Pro was a better fit for me until I lost it in a murky pond. Now why didn't I throw the Dragon on that one? Thing is, I haven't used enough different big D drivers so my take in this category hasn't got a lot of weight.