Mario:
His fighting style is pretty unique, taking elements from many old Mario games, including powerups like the Fire Flower.
His overall uniqueness gets pushed down by the fact that the majority of his smashes and aerials are pretty basic, conventional fighting moves involving many kicks and punches. 9 out of 11 of his A attacks have "punch" or "kick" in the name, seriously. But his specials are something else.
Fireball is plenty unique, a quick jumping projectile that doesn't require any charging up, and helps stun your fall if you want to attack in the air or get back to the stage a certain way.
Super Jump Punch is a good recovery, but nothing spectacular. It's a good reference, but it's pretty conventional.
Cape is a good move. It works like a reflector and hits a projectile back at who fire it at Mario. And while that is a unique move, it's a unique move that is shared among a few other characters on the roster.
Despite being the most hated move except maybe Falco's and Wolf's landmasters, FLUDD is one of the more unique moves in the game. It makes a completely different character appear on his back, and will stay for a while. The fact that he can run around and fight with FLUDD still on his back long after using him, is pretty interesting to me. And now that Squirtle has been most likely cut, there's no other move that shoots water like it. There's more things FLUDD does in Mario Sunshine, but they weren't used, so that can't help him, but it's still pretty interesting, so I think it helps his score.
His Final Smash isn't too interesting, it's not a copy-paste of an existing one, and does leave a cool fire affect afterwards, but it's not the best. So I give him a slightly above average rating.
B+
Luigi:
He started out as a clone. But has gotten some uniqueness throughout the series.
A lot of his A moves are similar to Mario, so I don't see much reason going into detail on them. They hurt his score.
I just want to talk about his specials.
Green Fireball is not very unique, but it's visually different, and looks pretty cool, but it doesn't work too differently.
He also has Super Jump Punch, but his he lands on the ground head-first, so that's something.
Green missile and Luigi cyclone are moves unique to Luigi.
His moveset shows off his personality very well, I'll give him that.
Luigi's new Final Smash takes the most important aspect of his series, and will most likely function the same way. A good plus.
We haven't seen it in action, but it looks promising from Sakurai's description.
Overall, he gets a C-
Peach:
Now this is a very unique character.
She has 2 unique mechanics.
Floating: She's a very floaty character already, but she also has the ability to float after her second jump, which can work very well as a fighting strategy when floating only slightly above a flat stage, going side-to-side around an opponent for a hit.
Move switching: Her side smash has her switching through 3 moves: a tennis racket, a golf club, and a frying pan... adding more variety to her moveset. Also, her down special includes many different vegetables, and certain expressions have certain affects (or at least there's one expression that's more powerful than the rest, can't remember fully)
Her side special is both hilarious and unique to only her.
And her up special used to be similar to an item, but that item has been cut, so it's now unique to her. and works well as an attack and a way to choose your speed when falling.
Toad works basically like a counter, when you try to hit her when she pulls out Toad, that... green stuff... sprays around to hit the opponent away. This isn't a conventional counter though, it's pretty out-there.
Her up smash is her twirling with a ribbon, her up aerial creates a rainbow above her, her up tilt makes her create hearts with above her with a swipe of her hand, and her forward aerial makes her attack with her crown. All unique unlike just kicking and punching. And pretty hilarious too, being as girly as possible.
I mean, if you wanted to make a moveset for Peach before she ever got in, would you have really thought of her taking her crown off and hitting you with it for a forward aerial? That's pretty out-there.
I think she might be the most unique non-team character pre-Smash 4
I'm giving her an S
No - or +, but a solid S
She went beyond the expectations of a Mario series fighter, just like Rosalina did years later, and has only become more unique with certain changes and the removal of similarities.