First, let it be known finchinator
has a lower win rate in nu snake games that hootie
at this moment in time.
1. Hitmonlee is a worse Passimian
2. It loses to Slowbro and Xatu or makes a lot of turns 50/50
3. Hitmonlee does not learn U-turn
Passimian is super common because it is splashable speed control that can also be a pretty solid Incineroar check and has uturn. Hitmonlee is a much softer Incineroar check that is not speed control (if you wanted to use LO you could get away with stuff like mach punch, though you shouldn't be using LO).
But this is not really what Hitmonlee is about.
Passimian is a hoodlum
. He likes ding dong ditching and trick-or-treating at the same house twice. He doesn't like staying in one place for a long time since the feds might catch up to him. He knows it's risky to get into a fist fight early on in the school year since it's really hard to catch up in math when you fall behind, so he waits till the end of the year before he brawls.
Hitmonlee is more like a unlucky firefighter
. He doesn't clock in until the end of the day when everyone is pooped, but since he just showed up he's full of energy. You can't blame him, though, his car broke down in the rain and he had to walk 8 miles to work. Poor guy. Really he needs all the help he can get, and he's so unlucky that you can't help but give it to him. But when the time comes, he's still a firefighter, so when a homie calls in and tells him it's time to go, it's time to go. He'll show up and fight that fire so fast you'd think he was a firefighter.
I hope these character sketches help paint a clear picture as to what the difference between Hitmonlee and Passimian is. Hitmonlee's only viable set as far as I'm concerned is
unburden curse as rabia said. This set has a niche, since the use of av bro on a lot of teams let's it clean up nicely a lot. If we look at common balance builds, a set of curse / close combat / knock off / stone edge or iron head cleans common teams and is pretty much impossible to revenge (you can see BU Passimian do similar things as far as coverage goes, but it's a lot slower in exchange for a zmove and uturn so Hitmonlee is more effective against teams with Rotom, for example). There's also no 50/50's involved when you aren't spamming HJK / choice locked attacks!
"Ok tangy, you've convinced me that Hitmonlee isn't totally useless. But if I run a mon with no mouth, then I can't run a monkey!", you say with arms up in dissatisfaction.
You'd be right, fighting spam is kinda poop since you can't really wear down the most common fighting checks (bro, ghosts, comfey). Earlier we said that Passimian is super common because it is an easy to fit Incineroar answer + speed control in the same slot. But let's say you have something like Rhydon + generic fast mon (think Whimsicott or Sceptile or Rotom or Heliolisk, etc). How much do you really need a Passimian? Rhydon + a soft check in Lee is pretty fine vs Incineroar for most teams that aren't 4 other slow, passive mons. Having a scarfer is drastically overrated since the only kinda relevant speed boosting sweepers are Scrafty/Klinklang (which both beat Passimian btw), and a mon like Sceptile or Whims can already outrun most of the tier (there's also Comfey if you're into that), and the really fast mons have some pretty solid defensive answers. Let's not forget that Hariyama is still up in the A ranks, and you really don't want to run Hariyama + Passimian either (though this is less of a sin).
sorry the last point is irrefutable, guess hitmonlee is unviable.
The point of my finchinator esque wall of text is not to save Hitmonlee from the depths of C
(though if you look at what else is in B, you'd realize it is placed just fine, though you could argue for B- I guess), just to point out some misleading ideas.
As far as Passimian to S, it's a no from me. In my eyes, Passimian is the only viable scarfer (scarf Rotom is mostly poop, not quite big poop) because it offers hard to deny role compression as an Incineroar answer + speed control, and is useful in all matchups because of U-Turn. But it does not warp the metagame around itself (being the best fighting obviously means it'll make other fightings worse so I don't think that counts, but stuff like Hariyama and Medicham are still viable). There are simple and common ways the meta has to deal with Passimian that it already had before such as protect or rocky helmet or colbur berry ghosts. There's a lot of things that live a hit and can hit it hard in exchange because of the -1 defenses from CC, and there's a lot of things that can punish you spamming U-Turn on them. Passimian is also often very prediction reliant, as being locked into CC vs an immunity can be a huge momentum suck. Really, I think it's a picturesque A+ mon, with low risk and pretty good reward.