I have a lot of favorites. Some of them are:
Chimecho, Gyarados, Absol, Ninetales, Ho-oh, Kyogre, Tropius, Breloom, Venusaur, Espeon, Furret, Pidgeot, Elekid, Charizard, Blastoise, Tyranitar, Metagross, Salamence, Blissey, Wailord, Jolteon, Dragonite, Latias, Aggron, Raichu, Typhlosion, Flygon, Swampert, Linoone, Mightyena, Swellow, Manectric, Gardevoir, Sudowoodo, Victrebell, Raikou, Suicune, etc.
BUT if I had to sum it up....
FERALIGATR is my ALL TIME FAVORITE!! :D