I definitely have to agree with Scythemantis on Gloom and Vileplume, and while they look like Scythemantis described them, they are kind which is what I like so much. (based on the episode in which Ash saves Erika's Gloom from the gym which is set on fire)
Another favourite of mine is definitely the Tyranitar line, I love the type, the concept (armour), the colours and the personality of Larvitar as portrayed in the Anime. I like defensive, heavy, bulky, sturdy Pokémon, and the fact that Tyranitar is Dark type as well, makes me like him even better.
I don't have one favourite Pokémon, but a few, because they are all different so you can't compare them. Another of my favourites is Forretress, also for the defensiveness, and because I love bug type. And I love his voice.