My top pokemon is probably Gardevoir or Espeon, but some of my other favorites are Ekans, Cyndaquil, Piplup, Snivy, Wigglytuff, Shinx, Giratina, and Darkrai
My favourites are from a mix of gens: I love Gardevoir, Lucario, Dragonair, Kingdra (shockingly!), Lugia and Ho-oh, Garchomp, Sylveon...although I started playing more casually after Gen 4, so I'm not as familiar with the newer mons.