The two best defensive fighters that Louis faced were Conn and Walcott. Let's look at Conn first.
Conn actually did not slip Louis's jab particularly well. He took it away from him, tactically, but Louis actually did well with this punch even in the stretches of the fights that he loses. You can see Louis land tentative jabs even in this video, put together to highlight how well Conn did. [Only registered and activated users can see links. ]
Throughout. He doesn't land it well here, but that's because Conn is keeping him off-balance throughout with the same tactics that he uses to keep the rest of Louis's offence under control. I think the jab was maybe the most regular scoring punch for Louis in this fight, and Conn slipping the jab is actually not that common, though he does parry a few.
Basically, in the worst fight he had for offensive fluency, he scored with it throughout.
Now Walcott: [Only registered and activated users can see links. ]
3:15 is a good example - Walcott takes this jab out of the way because he is trying to slip it. OR, the footage misleads and he slips it by a hairs breadth. Hard to be sure. But Wlacott learned his lesson early in this fight. He ends up going away from the Louis jab (4:49, 4:51, landed) rather than trying to slip it, rellying upon his superior footspeed to neutralise the jab. This is the crux of the Louis jab as an isolated tool. It was speedy, accurate, and, yes "thrown properly" (come on...). If you were in range, you'd it tended to land. Louis's other limitations meant that he sometimes had to use the punch in different ways, which was what he absolutely perfected in one of the rarest skills of all, which we'll get to.
You're a good guy. I like you. But this era is not your era of expertise and you've missed something pretty fundamental here.
This is intentional.
When I say Louis was a better counter-puncher than Toney, I absolutely meant it. When I said he was better at countering the space, this is what I meant. Louis, as perhaps the most concise puncher in all of boxing history has one job - walk his man onto his punches. How do you persuade your opponent to do that when you are known to be such a destructive puncher? You have to find ways to bait your opponent in in such a way as you're able to counter him. Louis often poked out slow, short jabs in order to bait his opponent in.
The fighter who came closest to taking advantage of this "weakness", after Schmeling, was in my opinion Nathan Mann. The Mann fight is a study in Louis. Louis has a weak first round. He scores with some good jabs, and paws randomly with others. Mann, really really cautious in the first minute, starts to become more and more aggressive. He starts to counter the Louis jab and actually lands some pretty good punches against Joe, in a similar, if cruder way than Schmeling did, taking advantage of that low left hand. But the low left hand is bait, bait in the space Louis wants Mann to recklessly inhabit. Very few fighters obtain this level of skill in this area. Only Hopkins and Juan Manuel Marquez are even comparable in this era, and they really weren't as good to my eyes, although Marquez was just as committed.
[Only registered and activated users can see links. ]
Anyway, watching this fight from the beginning is a grand idea. You can see Mann becoming more and more aggressive in contrast to the beginning of round one. If you want to see this point illustrated though, whip on to 4:15. Louis tosses out a jab that's nothing and Mann comes boiling in, totally out of control. Louis doesn't make him pay, but his opponents caution is neutralised (not least by his own success, admittedly).
You can see more lazy jabs bringing Mann on at 4:30 and compared the "snapped" jab at 4:53 with the lazy one at 4:56 that brings Mann onto the two-part combo that results in the first KD and the beginning of the end.
He also used slow, lazy jabs as flat out feints. Watch here at 4:52 as Louis tosses out a shitty lazy jab slightly wide to open up his man for what is actually an out-and-out lead right over the top: [Only registered and activated users can see links. ]
As a pathology, this is far more savage, complex, and skilled than anything Toney brings to the table. Louis's jab is not amongst the very best at HW, it doesn't have the variety or reach to compare with the very best, but as a tool, it's in the very second clutch. And yeah, better than Toney's.
Why bother when you are the best combination puncher in the history of boxing? Louis had a tendancy to land power-punches behind his jab. Jab is a tool. Incidently, this is also part of the reason I rate Lopez so highly with the jab.
As we've seen, Louis had far darker ways of creating range.