The last few episodes made me wonder how they did Uncle Iroh's voice after Mako died. They either got a dead-on impersonator or some mighty fine soundboard work.
Greg Baldwin did some great work filling in for Mako.
This series is definitely a contender for best animated series, but I'd put up Death Note, Fullmetal Alchemist, Batman: The Animated Series, and Justice League Unlimited in the same tier.