Picking just one -- out of dozens of blends I thoroughly enjoy -- is tough; what was #1 yesterday may not be my favorite today, given my mood, the weather, whatever. But my standard, go-to answer is Esoterica's Penzance. It's been at or near the top of my list since I first tried it in 2005, and I still enjoy it.
As for "why": I love everything about this blend. The flake is velvety smooth in the palm of my hand -- unlike any other I've tried -- and it breaks up effortlessly. The unlit smell is a rich combination of orientals and latakia. And, when lit, the blend is virtually seamless; it's very very difficult to pick out the constituent leaf. It all just melds together beautifully into a rich, uniquely spicey smoke. It is not heavy with latakia; the latakia fits neatly into a blend that also includes Virginias, orientals, and black cavendish. And for me it's just an incredibly smooth and pleasing smoke, with an oriental tang I don't get from any other blend.
Bob