Lucario doesn't stand out to me as a pokemon in general; I seem to be in the minority with this view (although I'm basing this solely on my experience playing Diamond so far. I haven't seen this movie that he's in). That said, I understand how popular he is for whatever reason that may be. He IS a fighting pokemon, he's got some Smash-worthy attacks from what I've seen, and he seems to almost parallel Pikachu's popularity at this point. Seems like the best choice to me.
I don't see the draw to Blaziken at all. I agree with Caael about the starter pokemon logic. How can you include one starter as a playable character without including it's 2 counterparts from it's game? What makes Blaziken more deserving than the others?
Meowth seems like a logical choice. In the games he's basically just another throw-away pokemon, but let's be honest—the anime counts. Otherwise, Pikachu would be no more fitting for a spot in Smash than anyone else, right? That said, I'm sure if Sakurai and the gang cared about that at all, Meowth would've been in Jigg's spot in the first Smash. I see Meowth as officially shunned at this point.
And Deoxys. Not to be superficial, but ****. What a hideous pokemon. I don't mean that in a "gross" or "scary" way. I just mean it's a BADLY thought out character model. It boggles my mind to this day how a character designer over at Pokemon HQ ever drew that and said "Yeah...good enough. Legendary even!" This is all, of course, in my humble opinion though.
That said, I see why people want Deoxys. It's one of the few legendaries that has enough human-like characteristics to be a playable character. Still, I'd much prefer Lucario. than anyone else.
And I'm sure this has all been said 23948239874 other times among the 145 pages of this thread, but I felt the need to share.