McGregor's obviously a great boxer, but I haven't seen anything from him of the same caliber as Aldo's jab, head movement, or ability to throw in combination. He'll get his chance to prove me wrong, though, and I'm excited for it.
JSD is an awful boxer. No movement, no jab, just an overhand right and uppercut.
While he certainly doesn't have "no jab," there's little doubt JDS is a pretty flawed boxer, especially his footwork. But to be honest, there have been plenty of elite HWs in boxing with some glaring flaws in their game. In the heavyweight division, JDS is probably the pick despite his flaws. Sergei is the only real competition, and he's more injury-prone than Cain.
At LW I would say the best boxer might actually be RDA. Pettis is a great striker, but as much for the kicks. Takeaway that and he becomes less fearsome.
Dos Anjos was my thought as well. Speaking purely of their boxing, I'm more impressed by what I've seen from Dos Anjos than Pettis. Alvarez is an interesting choice as well. Almost certainly the best offensively, but his defense holds him back.
WW is the other tight division. I would say Rory's boxing is a really high level. He is very technical and he controls distance so easily. Obviously Lawler has really technical boxing as well so he might be a tad better
WW is really tough. McDonald's a solid candidate. I don't put too much stock in the Ellenberger or Woodley fights, since they were both sitting ducks against a good jab and that's the best part of McDonald's game, but after a rough 2nd round he made great adjustments to close the show against a very talented (albeit chinny) Saffiedine, going well beyond his previous over-reliance on the jab.
Lawler pretty soundly outboxed Rory, so he's got to be a candidate. However, Rory may have improved since then, and a southpaw is always going to be a challenge for a jab-reliant guy like Rory. Lawler's got better movement and better defense, though.
Then there's Lombard. When he's on, I think it's him. Excellent speed. Moves well. Works both the body and the head with beautiful combinations. A more well-rounded and versatile boxer than either Lawler or McDonald, I think. No major glaring weakness in his defense. More stopping power than anyone else in the division. His mental game and consistency are big questions, though, and I'm not inclined to call somebody the best just because he peaks the highest. Consistency matters.
Conor is not a great boxer, he keeps his hands down most of the time.
Lots of great boxers keep their hands down much of the time, and that's without the threat of the takedown. Keeping your hands up all the time is a great habit when you first start boxing, but it can also be a great one to forget once your defensive bag of tools gets bigger, since it can fatigue your arms needlessly and make your offense predictable.
