You forgot to add Re:Coded into the polll, since it came out in 2011. But, I digress.
My favorite Kingdom Hearts game of the decade is definitely III! Over a decade's worth of buildup and hype, with stories and characters and emotions all leading up to a grand finale was one of the most satisfying experiences in Kingdom Hearts I've ever experienced! Kingdom Hearts III was the epitome for me, and I felt immensely satisfied with the conclusion to the Xehanort Saga! I couldn't be happier to end the decade than having been able to play through the whole Xehanort Saga! Thank you, Nomura and friends! You really hit it big!