Mega Bidoof
Smash Hero
I agree with the putting aside personal feelings.I like this thread. I think you're going to have a hard time getting people to not vote on who they want more, rather than putting all personal feelings for the characters aside and judging uniqueness alone, but it's worth trying out anyhow.
It was a relatively hard choice for me amid the Mario choices, and it eventually came down to Paper Mario, Petey, Fawful, and King Boo. I ultimately chose King Boo, because he has so many great options from his boss fights, ranging from Boo creation and manipulation, general invisibility, levitation, materializing of objects, and possible possession, and having the most unique stature, constantly floating off of the ground.
Petey and Fawful have a diverse collection of attacks and gadgets, respectively, of their own, so they're probably interchangeable between second and third, while Paper Mario might have been higher before Rosalina, but the fact that she already uses a puppetmaster gimmick leads me to look less fondly upon Paper Mario's helpers. Otherwise, Bowser Jr. and the Koopalings have too much semi-clone potential, Waluigi would be yet another prop abuser, E. Gadd's most prominent weapons have functions already seen in Smash, same with Birdo's few attacks, and the rest are self-explanatorily disregarded.
But the again, most people like the characters they like because they feel they are unique.