I think the only non-stock airplane that have is a repainted King-air, and I'm probably going to switch it back. But then again, I'm probably going to re-install my FFG Learjet 35s.
To answer the question, the default airplane that I spend the most time with is the Learjet 45. I like the Mooney (IFR panel), and the Cessna 182 S. And the Baron is another one of my favorites.
I don't spend much time with the airliners. Mostly I just like to see them as AI traffic. But when I do fly an airliner, it's almost invariably the Orbit 737. I also have been known to fly the Emerald Harbor Caravan.
It'd probably be easier to phrase it in terms of the default aircraft that I don't fly very often. And the answer to that would be:
The 747 and 777
The Schwiezer (which I intend to spend more time with later)
The Caravan floatplane