Pre-injury Martínez still couldn't win aerial duels, cover big spaces quickly, organize his defense around him, or pin down attackers. Maguire does all these things with ease except the big spaces part. He also steps into midfield to aggressively intercept incoming passes to the opposition attackers. In possession, he's better than Maguire though, I'll obviously not try to deny that.
Varane has always struggled in a high line, and in the build-up. Couldn't pass progressively, couldn't carry the ball, and was a liability under pressure. His best performances and moments at United came when he had to defend his own box because we were getting dominated and had to defend deep. He's probably a better box defender than Maguire, but that might be the only part of his game that he edges over the latter. Even his recovery pace was going towards the end of his time here...but he did well to babysit Martínez and help him, and they played more together. In possession, Maguire's always been way better than him.
Regarding Varane and Martínez, it's also important to consider that they formed one of the deepest lines in the league in both of the seasons they spent together at the club. In the two seasons under Ole that we had Maguire and Lindelöf as the two starting CBs, we played a higher line, played more proactive football, but still conceded less goals, despite taking more risks, like I said. And that was without Onana, Dalot, and Martínez, three players that are definitely way more suited for the build-up than De Gea, Wan-Bissaka, and Lindelöf. Ole only really had Shaw and Maguire who could excel in a high line, and in the build-up. Ten Hag had better tools at his disposal, but he just benched Maguire and decided to play a deep line all the time.
With Lindelöf, I don't even think I need to say anything. Good passer but it's quite a feat for Ole that he achieved back-to-back top 4 finishes with Lindelöf as one of his starting CBs.
De Ligt's been bang average for United IMO and he's awful in the build-up. No progressive passing, no ball carrying, and doesn't provide any calmness in the build-up like Maguire does, either. He can't wait to get rid of the ball and it's always just a sideways pass or back-pass to Onana.