Personally, I'd argue against ZSS's jab and put Fox, Lucas, perhaps Mega Man because of his unique jab, Ftilt, and Nair, or even Robin solely because of how powerful and well-connecting her rapid/wind jab is while fire jab just outright kills given the chance. Why? 'Cause ZSS's jab only has speed as a factor. Range, coverage, and damage? Her jab's pretty mediocre since she can't use really jab to grab due to her slow grab and weak knockback. I don't know why, but for whatever reason, her jab 1 will whiff on shorter characters while I think Ryu's seems disjointed vertically letting him cover say a Pikachu running. This is despite the fact both of them jab pretty high up compared to say, Ganondorf who despite being tall, slightly below his shoulder instead of doing a backhand above shoulder level.
Why people say Captain Falcon's jab is for multiple reasons: one, it is fast. Yes, there are frame 2 and below moves, but frame 3 is still pretty fast, hence, why people complain about frame 3 aerials breaking combos by mashing. Two, Captain Falcon's jab 1 refresh - some characters can hold down attack and continue jabbing in place - is among the fastest with Little Mac. The other one is that each part of his jab lingers longer that other jabs. For instance, Fox's jab 1 is frame 2, jab 2 is frame 2-3, and rapid jab finisher is frame 3-4; that's only 1 active frame for jab 1 and 2 for the rest of his known jab data. Little Mac's jab 1 is frame 1, jab 2 is frame 1-2, jab 3 is frame 4, and rapid jab finisher is frame 5; he only has 1 active frame per jab part. Captain Falcon's jab 1 is frame 3-5, jab 2 is frame 5-7, jab 3 is frame 6-8, and rapid jab finisher is frame 6-7, so jab 1, 2, and 3 have 3 active frames and rapid jab finisher has 2. Similarly, Ryu's jab 2 and jab 3 have very high active frames; jab 2 is frame 3-5, 3 active frames, and jab 3 is frame 8-10, 5 active frames, but what matters is the initial contact, jab 1, which for Ryu is only frame 2-3, a "normal" 2 active frame jab. When Captain Falcon jabs, especially jab 1, he pulls back fast. Animation-wise, that's a "proper" jab if you're talking about it in terms of boxing. There is no "posing" like what happens when Falco and ZSS jab. At the same time, the higher active frames means that even though he pulled back, he might actually still have a hitbox out - yay, more Captain Falcon hitbox shenanigans. Note: none of this includes transition frames which is when a character's jab combo can transition at the earliest. So, even though Captain Falcon's jab 2 is frame 5, he might be able to transition from jab 1 at frame 4 making it a very fast transition compared to say.
Now, the other part is comparing it to other jabs, particularly, how they connect. Captain Falcon has always been known for having a well-connecting jab since 64? or Melee at least. Other characters? Not so much like Marth's jab was so-so until patch 1.0.6? if I remember correctly changing it so it was more vertical letting Marth followup more easily since his jabs are 2 uppercuts with his Falchion. Or perhaps Diddy whose jab was dysfunctional because rapid jab wouldn't connect properly meaning characters would fall out or just not get hit. Even Ike, who was known for having a good jab in Brawl, ended up with a crappier jab in Smash 4 until patch 1.0.8 re-tuned it to connect better and transition faster like in Brawl, but not as fast as or crazy as Brawl.
You mentioned that you believed Falco's jab was good and better than Captain Falcon's... It's not... The transition from Brawl to Smash 4 was harsh on Falco's jab despite it not being an abusive move at all. +7 frames were added to both jab 1 and jab 2's total frames meaning Falco can barely manage jab mixups without getting punished and, for whatever inexplicable reason, his rapid jab transitions slower from jab 2 - about frame 2 to 4. This more or less means that Falco's jab does not connect well when doing full jab combos, the higher recovery means jab mixups are risky, and I'm going to assume hit angle changes were made since unlike other rapid jabs, Falco's pushes you forward very quickly unlike Fox and Captain Falcon's where it's slower or Palutena, Robin, and Rosalina's where you can barely move. Unfortunately, rapid jab frame data is barely covered anywhere, so I don't know, but I can probably assume that Pally, Robin, and Rosie's jabs probably have auto-link angles or higher? - or is it lower? - SDI multipliers assuming SDI affects rapid jabs. To make matters worse, Falco's rapid jab has a narrow hitbox and despite the Pits having narrow rapid jabs, theirs is disjointed; Falco's isn't. This means that Jigglypuff can Rest during Falco's rapid jab because of the I-frames on Rest's startup combined with her making contact with Falco's hurtbox. Hitbox-wise, it's shoddy enough where Little Mac, a fast faller, can jump out easily, Lucario can land in between his jab 2 and rapid jab, and Captain Falcon, Luigi, Mario, and ZSS at low percents lands fast enough they can jab or grab through Falco's. The time between his jabs are all negative on-hit if you choose not to go through a complete jab combo. It's to the point where theoretically, at around Mario's weight, anyone with a frame 8 and below move can punish him, though, in practice, it's more like frame 5 and below. There are hard punishes that are below frame 8 and 5 like Ryu's Shoryuken, Yoshi and Luigi's Nair, Roy's grounded Blazer which has armor unlike on the ground, Luigi's Super Jump Punch, and pre- and post-patch 1.1.0 Luigi's D-throw combos/setups. If they aren't hard punishes, they're still going to get more mileage or better positioning like Mario doing a Nair or Super Jump Punch will definitely net more than 3% to 5% from a failed jab combo or mixup from Falco.
Rapid jabs having poor connections isn't limited to Falco. It involved Diddy as mentioned before, Greninja, Captain Falcon, and even Fox. Difference? Hitbox coverage like Greninja and Captain Falcon's being larger, different options like Greninja and Captain Falcon can choose to do 3-hit jab combos instead of rapid jabs or choose to do incomplete jabs more safely because of lower recovery, Diddy's getting fixed, Fox's gaining a phantom hit between his jab 2 and rapid jab, and how they hit or what kind of hit like the Pits being disjointed so even if people fall out, it's not going to be easy to just clank or hit through them.
Oh, and one last thing, for the love of all that is good and boxing, developers, please make it so Little Mac's jab input is the same as everyone else. Yes, we know his rapid jab is awesome, but I'd like it if I can input his 3-hit jab without accidentally going into rapid jab. Hell, this "homogenization" would actually fit with Little Mac being a boxer: boxers don't commit unless they know they're going to get more than one hit. There is no reason why Little Mac's "natural", hold down A jab is a rapid jab instead of his 3-hit jab. Also, please make Shulk's jab at least frame 4. It's not freaking rocket science on how to punch. Seriously, watch how Shulk jabs. He actually looks confused if you don't go into jab 2 and the time he takes to jab also makes him look confused on how to attack.