Lucario really just makes a lot of sense to be added. Already popular, they pushed the name early (movie, mention in Mystery Dungeon) and of course, representative of the new generation. Much in the same way Roy was there to hype people about the upcoming FE, many people who play Brawl might be inclined to buy DP because 'Lukaro amm TEH k00l'
Despite this, the question was who deserves a spot in Brawl most, which made my vote Meowth. All of the Pokemon so far have had significance in the anime, Meowth has been around for a long time, and his exclusion in Melee caught me by surprise (other than them not wanting to have only first-gen Pokes). I can't think of any other Pokemon in the anime who has had much more impact (nearest is Togepi, who was at best filler).
Everyone likes to opt with their favourites, but when it comes down to who deserves to appear, it's a matter of logic. I wouldn't vote for Venusaur or Cubone soley because they rock, give me a reason. Cripes in that sense I'm practically anti-Blaziken.