For me, it's the BK15 and the BK4. These are my 2 main blades when I'm in the woods.
I started with a BK2 and a BK9. These are both great knives, but for me the BK4 is almost a hybrid of them; it weighs the same as the BK2 and has nearly the same chopping ability (possibly slightly better once you master the recurve techniques) as The King. I still have my BK2 and my BK9, but they don't go out nearly as much as they used to.
I also tried both a BK11 and a BK14, but they were really too small for my hand. And if I'm gonna use a blade that small, I'd rather just use a folder for easier carry.
So my 3 main knives are my Spyderco Tenacious folder, the BK15, and the BK4. The folder is my EDC and gets used most often. I use the 15 for food prep and for carving and bushcraft. The 4 is used for bushcraft and wood processing, it's the perfect woodsman's blade.