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.
Electivire is Awesome! So is Garchomp, I liked Gallade's type combo and Magmortar as well. Lucario, Rampardos, Bastiadon, Darkrai, Dragon Trio, Arceus, Togekiss, Regigigas. Lots of good pokemon. Mostly legendary and new evolutions of old pokemon