At first I liked Blastoise the most, in every Pokemon game i used to choose Squirtle as my starter which will become Blastoise in future.
And when Pokemon XYZ Anime came and when I saw the last evolution of Froakie which is Greninja, it looked way cooler than any other Pokemon till now I've seen.(except Infernape, because I love Water types). And also after Frogadier, the Greninja which we get is Water as well as a Dark type. Who doesn't like Dark type?? Because till Greninja was introduced the most beloved Dark type was Lucario in my opinion!!! So here Greninja became my favourite.
And at last Gardevoir, because of the new Fairy type, plus Psychic type. And all must have already known that why people love Gardevoir ;)
When I was younger and was in Pokemon for a little bit, I don't think I even had a favourite Pokemon. After I was reintroduced into the series, it was Houndoom because I loved dogs and it had a slick design. Cue Gen 5 and I just fell in love with Scrafty after 6 months of BW's NA's release date and I don't see that changing anytime soon.
While my #1 doesn't change, my top 10 sometimes fluctuates.