Congrats on the YS640 GRAVYDAWG! Have had a YS640 since the January before last and can tell you your going too love it!
From my experience Lumberjack and BBBQr's Delight are the two best choices available. I use Lumberjack myself and sell them locally, below is a breakdown of how Lumberjack's pellet flavors are manufactured.
Hickory - 100% Hickory Pellets
Oak - 100% Oak Pellets
Cherry - 100% Cherry Pellets
Maple - 100% Maple Pellets
Apple - 60% Apple/40% Oak Pellets
Mesquite - 60% Mesquite/40% Oak Pellets
Pecan - 60% Pecan/40% Oak Pellets
MHC - 1/3 Maple, 1/3 Hickory, 1/3 Cherry (this is a blended pellet, not a mixture of pellets)
OHC - 1/3 Oak, 1/3Hickory, 1/3 Cherry (this is a blended pellet, not a mixture of pellets)