As a Kid (Basic): I don't remember that well (believe it was Squirtle and his/her evolution line though).
As a Kid (Legendary): I believe it was Mew (would've been Mewtwo if I didn't think he was a jerk back then. The events of the 1st movie didn't really help him leave a good impression on me).
Now (Basic): Mudkip and his/her evolution line (It's not only cute, but at its final form (Swampert) it can be really powerful if you give it the right moves).
Gyarados comes in an close 2nd. The only thing I don't like about this pokemon is his/her 1st form Magikarp and the fact that you have to evolve it into this pokemon at level 20. Once you get past that, (s)he's not bad to have in your team.
Lucario comes in an even closer 3rd, but 1st impressions told me that he's not exactly the good guy when I 1st saw him in his movie. Me not being a fan of the good guys being pushed around for seemingly no good reason didn't really help him out until I checked out the summary for his movie.
Now (Legendary): I would say either Mewtwo, Lugia, Rayquaza or even Mew.
I had another favorite in the days of Ruby/Sapphire and Emerald (mainly Emerald) and that was Jirachi. Not sure why I liked him/her then, I guess (s)he left a good impression on me.