My all time number 1 is Sandslash Numero 2 is Whimsicott
Nummer 3 is ....... well the bunch isn't ordered that much
My remaining faves are Exeggutor, Volcarona, Sigilyph, Amphabulous, Starmie, Eelectross, Arbok, Onix (as of now, I don't know what gen 6 will bring. And all original 151 will have a special place in my heart)
So: Nummer 3 is
Ampharos alone was already one of my favorites, but I really fell for its Mega Form, even though it was really random and strange at first sight. I chose it mainly due to the "Would I want it as a pet" factor and not the "you are so badass in a fight" factor: While I love love love Starmie, Onix, Volcarona and Exeggutor, I don't see them as pets.
While I don't despise them, there is really no Pokemon in Gen 3 or 4 that blows me away with its awesomeness.