Cristiano Ronaldo - unique for multiple reasons. First of all, it's his skillset. When he entered the scene, he was a fancy and spectacular but ineffcient dribbler/trickster without much end product. The sort of player who'd secretly roll his eyes in training when his coach adviced him to become better at something "down to earth" like heading. Yet he became arguably the best ever at it. There's probably no player of his quality that get's the basic as right as Cristiano. And that's the foundation for his second unique characteristic, the position. There are quite a few players who score many goals from playing out wide. Salah, Mbappe, Neymar, Robben, Sterling, Messi and so forth. But that's not really comparable to what Ronaldo does, who often effectively plays like a striker, demanding crosses and muscling off centerbacks. And last but not least, there's no other player I can think of who transformed himself that immensely over the years. I mean, the player that won his first CL with United is so incredibly different from the one that won his fifth with Madrid, and he was one of the two best players throughout all those years regardless.
Quaresma - in the beginning, Quaresma and Cristiano were quite similar in style, IMO. Many of Cristiano's skill moves were reminiscent of Quaresma's and vice versa, but Quaresma - while being a far inferior footballer - retained this playing style. And on top of that, he's got probably the best outside of the boot in football history. He's probably the only right footed attacker that plays as an inverted winger on the right wing. If you wanted to play with crosses, you'd position him on the left, which is crazy if you think about it considering he's right footed - solely because he's better with the outside than with the inside of his boot.
Ronaldinho - has been explained here numerous times. Nobody was able to do what he did, especially on that level. Probably the most talented footballer in terms of ball control ever - and my favourite footballer of all time.
Ronaldo - probably doesn't need explanations. Incredible player
Thomas Müller - again someone who's named quite often in here. I'd never have thought that somebody with such a fluky technique could become so good. Sure, there was Inzaghi who scored many goals as a poacher but Müller isn't/wasn't that sort of player. He was no fox in the box who managed to escape his markers but someone whose runs all over the pitch were immensely important for the build up of attacks, too. Really shows how good you can become by getting the basics right.
Marcelo - full backs have become incredibly important in the last 10 years and their role has changed immensely. Nowadays they are often the only wide players. Dani Alves is the role model, a good technician, passer, crosser and tackler/defender but his standout characteristic was his athleticism. You'll finde many similar players among world class full backs in that time frame: Maicon, Alaba, Lahm, Alba, Walker, Robertston, Cole and so forth. But not Marcelo. He's nothing special from an athletic point of view, but his technique and passing is incredible. There are very view attackers or midfielders who can claim they have a better technique than him - if any. But there's not a single defender that technically even came close to him, let alone a left back.
Lucio - I've never seen a centerback like him when he was at Leverkusen and in his early Bayern days, honestly. He was notorious for constantly picking the ball up in his own half and just sprint forward with it, at times unstoppable like a train. Scored many goals that way (21 in 122 games for us), even many long shots, at times from free kicks. Van Gaal and Mourinho later forced him to abandon this playstyle for obvious reasons, but he definitely was unique. Here's a video if you are interested, really interesting if you haven't seen him:
Aguero - sure, you don't need to be big anymore to be a great striker and score many goals but honestly, most of his peers play either as false nines or as second strikers, like Griezmann or Dybala. But Aguero is different. I can't remember any other player that was able to play as a lone striker with his height and weight. If you take a look at the world class strikers played as single no. 9, then Aguero stands out because he's probably 15 cm smaller than their average. Whenever people debate about a striker transfer target being to small, someone will eventually drop Aguero's name.
Alexander-Arnold - Beckham is named often in here for being probably the best crosser the world has ever seen and that's fair, but I think Alexander Arnold is an even better shout because his crosses are most likely the best since Beckham has retired and he's played as a full back. Like Marcelo, he's completely different to his world class peers in that position. Although he's also very athletic, his crossing is probably one or two levels above the second best (Kimmich or Marcelo, I'd say).
Cannavaro - like Aguero, but as a CB. He's so unique that likewise to Aguero and strikers, whenever people debate about a CB being to small, someone will eventually drop Cannavaro's name.
If it's really solely about uniqueness and not about being extremely good at certain things, I'd probably go with this eleven:
Aguero
Ronaldinho Müller Quaresma
? ?
Marcelo Cannavaro Lucio Alexander-Arnold
Chilavert
Can't really think of unique midfielders, honestly. It seems like everyone I that comes to mind has someone with identical style.