Sanji's so called superior coo is good only when he's simping.
His over all haki stats sucks and we are in the final saga not in FMI.
How does Observation Haki even work?
The most common usage of Observation Haki allows the user to sense the presence of others. Rayleigh states that fully mastering this ability would allow the user to see others, even if they are concealed from view or too far to see naturally. This type of Haki allows the user to see another person through their aura, be it behind buildings, strategically hidden, or camouflaged.
Given this, Observation Haki, depending on the level, can discover who is hidden, but still present. When viewed this way, the user cannot actually see the person or animal with any physical characteristics but rather recognizes the target(s) by their "aura", which is a mental image of a colored silhouette of them on a dark background, as demonstrated when Usopp sniped Sugar, or how the blind Fujitora sees the world. Characters who use Observation Haki often describe presences as "voices" and can usually tell if specific people are still alive, dead, or close to death depending on whether they can still "hear" them.
There are other uses for observation, such as sensing the emotions and even the level of strength of others, but for the sake of this post, I will be focusing on this type of Observation Haki.
Out of all of the types of Haki, Observation seems to differ the most from character to character and be closely tied to the person using it, instead of Armament which is more standardized.
The way characters use it also depends greatly on their fighting style. The best example of this is Usopp.
Usopp's sensing of auras at a distance and ability to shoot is impressive, and his use of Observation Haki may be specialized for observing and hitting targets from great distances due to the way his mind works, which is influenced by his fighting style.
Sanji's Personality and How It Reflects in his Haki
Sanji as a character is someone who is very chivalrous. Interestingly enough, chivalry, which is a sort of code of conduct, is something that originated in France, the nation of origin for Sanji if he were from Earth.
Usopp getting a whole continent is wild.
While all the values of chivalry don't necessarily apply to Sanji, a good amount of them do, such as being generous, protecting the weak, and most importantly duties towards women, which includes serving them and protecting.
When it comes to women, this devotion is increased two-fold due to his perverted nature, something anyone who reads One Piece would be familiar with.
We are also told through Rayleigh that most people have a tendency to specialize toward one color of Haki.
Sanji is also confirmed to specialize in Observation Haki (specialize =/= best at it by the way).
How does this all come together?
With the way Haki works on an individual basis and the way women are so deeply tied into Sanji as a character (in more ways than one), I think it makes sense that his Haki is stronger around women, particularly when they are in danger.
Sanji also shows good Haki feats outside of women being involved, so to call his Observation Haki feats gag feats doesn't really make much sense. Especially when other Straw Hats are present, and they can't sense what he senses (even when pointed out to them), so, therefore, his Haki must be better in that sense.
In the three panels, we can see that he pretty consistently shows good observation. In two of the examples, he shows that he can sense a bit more than Luffy at least.
In Chapter 996 (the Wano panel) he picks out a conversation from a specific floor and Jinbe and Luffy aren't shown to hear anything (Luffy also implies Sanji is stronger in that area of observation in subsequent panels). And in 1060 (the last panel) he not only notices the lady in the water (Luffy does as well, presumably Zoro too as he accurately aims his slash) but he also senses her screams.
This extra bit of detail, especially in hearing voices and even teardrops as shown above, is something that only Sanji and Enel have shown, with Enel having his Devil Fruit augment his observation.
Sanji's observation feats concerning women are not gag feats, but more than likely reflect his character which undoubtedly plays a large role when it comes to Haki. These feats are pretty consistent, and he has also shown good feats in observation that don't strictly have to do with women alongside also being stated to specialize in it, so it's only natural that his observation is powerful.
Who Has More Range (between Zoro and Sanji)?
Range-wise, they should be pretty comparable. At the end of Wano, both of them alongside Luffy and Jinbe seem to be peeking in on the Scabbards + Yamato fighting Greenbull.
They also similarly "sense" the Gorosei arriving on Egghead while both are similar distances removed from the situation, so both their feats are relatively similar. Most their feats in this sense seem to be similar, except for one.
Chapter 1044.
Here Sanji in his sleep can sense Luffy (presumably his Haki) as he transforms into G5 for the first time.
Map of Onigashima.
The red dot with the red arrow pointing towards it is where Sanji was at the time, the Left Brain tower, which is directly across from the right brain tower, where Franky is fighting Sasaki at the time of this map's creation (Chapter 1004), and where Zoro would later fight King.
Luffy is all the way on the Rooftop when Sanji sensed him. As a reminder, Onigashima is HUGE.
That tiny red dot is the Thousand Sunny. Kind of ridiculous once you look back at it.
Anyways, that's most of their most recent feats for range. If you acknowledge Sanji's women-centric observation feats and don't view them as a gag, he kinda clears in this aspect but even without that they are more or less equal as you can see. I would give Sanji a slight edge though based on this feat.
Combat-Related Observation
In terms of combat, observation definitely helps with reaction and combat speed. How do Sanji and Zoro measure up in that sense? It's incredibly hard to compare this, mostly due in part to their wildly different fighting styles, but let's attempt something.
For Zoro, we have two relatively recent feats.
He can cleanly perceive and react to King, although he does unfortunately get tagged but I daresay that is entirely due to the massive size difference, and Zoro seems to plan to eat the attack in exchange to land his own.
Here against Lucci, he also cleanly evades all of Lucci's finger pistols before clapping him. Although we don't have the most concrete frame of reference for Lucci's speed, he is definitely pretty fast.
Two pretty decent feats. For Sanji, since his fighting style involves him zipping around his opponents for the most part, we don't have many examples of him reacting to attacks that are very usable, so I had to go through a different route.
Thinking about Sanji's observation, if it didn't scale to his movement speed, he wouldn't be able to control his speed and would end up like Captain Kuro. Let's look back at SBS 42:
Which is faster, between Captain Kuro's "Shakushi" and CP9's "Soru"? TELL ME. by Niku-oh
O: Ah yes. I got a surprising number of questions about this one. Captain Kuro's Shakushi (introduced in Volume 5) was a pretty tough one. To answer the question, speedwise they are about the same. But when Captain Kuro uses his trick, the speed is so great that he doesn't even know who he's attacking, but CP9 has complete control of their move at the same speed. So if there were a competition, CP9 would be overwhelmingly stronger.
Captain Kuro was as fast as CP9 agents using Soru, which gave the stronger Straw Hats a decent amount of trouble many arcs later, but he couldn't perceive at a level on par with his speed so he essentially wasn't in control.
Logically speaking, and tying this in to the in-verse example, Sanji would NEED a high level of perceptive ability to be able to fight at those speeds, and he does that in a perfectly controlled and utterly silent/undetectable manner.
Chapter 1034.
This speed is both feat and portrayal wise undoubtedly above what King did, and Sanji also has some pretty nuts feats even in Egghead such as outrunning lasers, so this is consistent. Scaling this to his perception speed which fringes on his observation, he should AT WORST be in the same ballpark as Zoro.
Final Verdict
Most forms of comparison we have between the two put them in the same ballpark when it comes to observation, which makes sense.
Taking into account Sanji's affinity for observation, and the feats that he has in regards to women (which definitely should not be dismissed as a "gag"), I would give him a slight edge, as now that I think about it, outside of future sight/regular observation being two separate things, I don't think the other aspects have as clear cut distinction, so it would make sense for someone who's better in any one non-fs category to be the best overall, and also throughout the course of the story Sanji has more feats and from what's shown is the better user, if that makes sense.