Very hard, I like a lot of Pokémon. If I have to choose, Garchomp (and Froslass, and Togekiss, and Roserade, and Lumineon, and Mismagius, and Drifblim, and Floatzel, and Luxray, and Shaymin, and Manaphy, and Arceus).
I can't explain it, I don't know why, but Garchomp is my absolute fave 4th generation Pokémon.
My fave of all time is Milotic, but Milotic is 3th generation and not 4th generation.
Milotic