I would say the first generation starter pokemon and their evolutions are the most popular in general, when we're not including pikachu. But when it comes to drawing and human-pokemon versions *shudder* the eeveelutions are the most popular.
I would have to say Eevee and it's evolutions. I could also say all Starters (at least to a degree-not all starters are welcomed *Fennikin and Oshawott for example*) and maybe even all the "cute" Pokemon (cute as is the ones that are constantly drawn, like Drifloon, Pichu, Goomy, etc) as well as Legendaries.
I was originally going to say Pikachu xD I guess the most popular would be the original starters. Like Squirtle, Charmander, and Bulbasuar. They usually end up being your strongest since they're with you since the beginning.
Charizard and its pre-evolutions are pretty much on par with Pikachu. All the other Gen I starters are pretty popular too, even among non-fans. Here's a quick list of popular favs: Ninetales, Eevee (and Espeon, Umbreon, and Sylveon), Ponyta, Fennekin, Lugia, Mewtwo, Articuno, Entei, Zorua/ark, Mudkip, Vulpix, Blaziken, Jigglypuff, Cleffairy, Staryu, Onyx
S'kindda hard cause everyone is different lol. I tend to favor a lot of fire types and dark types like Ninetails and Absol. I also LOVE Dragon types like Hydregion. As funny as this is, my boyfriend adores the new Fairy type lol. Even when we played Yu-Gi-Oh! together he had an entire fairy deck! Sooo...to pick one is hard but for me a personal favorite is....Eevee! <-- Totally an Eevee trainer lol.
My fav from the last gen is Delphox. I named him Time, haha. I grew very fond of him.
And in general, I love Haunter, Bulbasaur, Darkrai, Snorlax, the female Pikachu (That heart tail gets my gay side ON) and... I think that says about everything. I also love Vivillon xD She's my "parasite" as I call 'er, haha.
I do love the heart shaped tail female Pikachu! It is sooo adorable. Aaaah Darkrai. I liked him in platinum diamond and pearl. A misunderstood dakrness. Really like him. Haunter is also a good one a lots of fun. ^^ Bulbasaur is a little cutie to me too and deadly if you know how to use one. I like munchlax more than snorlax. XD I just like the cute chubbieness.
I have a thing for foxes though...they're just my favorites. I loved Delphox. ((he's not fire fighting thank goodness!))