If Medusa and/or Hades gets in over Palutena, I would be very upset.
Palutena has been in every Kid Icarus game yet, and has the games name after her in Japan.
Medusa was in two games, although one, as she was a fake in Uprising. (Whoopsie, spoiler)
Hades was the main villain of Uprising, but only appeared then. That's it, no other KI games.
Palutena is incredibly important to the series, not to mention the fact that she has more moveset potential than the other two (my opinion), and doesn't have to be resized or changed for Super Smash Bros. When you fight her in the Uprising boss battle, she uses light projectiles, counter attacks, and is only a little bigger than Pit (maybe 1.5x his size). I'm not saying the other two can't work but Palutena would not only be interesting, but easy to incorporate into Smash.