Quick summary:
Melee: Bowser, Peach, Mewtwo, Marth where the most highly requested characters. Marth not specifically but as the face of Fire Emblem it's logical. Ganondorf and Zelda where up there too.
Brawl: Sonic was by far the most requested ever here. Followed by the likes of Wario, Diddy, Dedede, Meta Knight, Pit and even Ike. Others who had decent popularity are easily Lucario, Lucas, Wolf and Toon Link. Brawl had a lot of fan favorites added.
Smash 4: Greninja, Robin, Lucina, Little Mac, Mega Man, Palutena, Pac Man, Villager, Mewtwo, Roy. These where notable ones. Most popular overall where definitely Mega Man and Mewtwo. Duck Hunt had it's fans too even.
Source: I was around bro.