*cracks knuckles*
This is an interesting question to wake up to! I’ve only ever wondered what Sam vs Sherlock would be like, and then John vs Dean, but those are for another day
Cas vs Sherlock... Let’s dive into it
So as Lexi has pointed out, there are some huge differences between the two. Additionally, we have to decide what contests are taking place, so I’ll mention a few I’ll think of. First, however, we should list a few strengths of our lovely boys to lay some base ground.
Keep in mind this is me ranting, and it's subjective!
Castiel
- a celestial being 400 million years or so old, give or take
- trained warrior
- can kill a bitch with a snap (notice that he can do so without the Infinity gauntlet, Thanos has nothing on him)
- skilled at strategy - I mean, I guess so, he was a leader of a garriot for a time, that's not an easy job, or wasn't
- has an angel blade to cut a bitch
Sherlock
- smartass since we can remember
- he can fight, obviously, but what style? Bartitsu is an option, but he's also a skilled boxer and he doesn't give up
- as far as 'superpowers' go, he has deduction as Lexi has said above, so this gives Sherlock a bit of leveage
- Sherlock is fit and agile, and since he spends a lot of time out in London (or wherever you have him, whatever AU fits you), he has some endurance worth noticing
- he's also a little shit who doesn't let opportunity slip past him, so he's quite creative I imagine
So, that's a few basics out of the way. However, we should now look into different fight scenarios, because we need to consider various aspects of Sherlock and Castiel
Fight Scenario #1
- Cas is an angel yet to be yeeted to turbo hell from Heavens, a.k.a. early season 4 Cas vs season 1 Sherlock
Okay, so this is one possibility that popped to mind. Season 4 Cas is a bit rough around the edges before Dean starts getting to him, and he doesn't have much patience, or rather, understanding for petty human bitch fights. Sherlock has a big bitch energy we all know and love, and him from Season 1 meeting Cas would be interesting. He'd try to deduce everything, but Cas is pretty much an enigma, plus Sherlock is most likely an atheist, so he'd fight having stroke first.
Physical fight wise: Cas would win. Grace is in his celestial veins and he doesn't fuck around. Remember that Dean punched him square in the face and his hand hurt, and Dean's a strong guy. Sherlock would have the same 'oof' reaction, and then he'd get either pacified or thrown across the room if he bothered Cas too much or tried to start something, essentially getting his ass whooped.
Deduction wise: I think it's a tie. Sherlock is a brilliant consulting detective, he observes, but Cas in a human vessel is odd even for angel standards. Remeber how he stood in one place in s4 and talked? Unused to a vessel, he has little to reveal, right? I believe Sherlock would have a bit of trouble assessing the angel, but he'd get something. However, let's remember that Cas can read your mind by just touching your forehead. He'd get a read of Sherlock faster.
Conclusion: Cas would likely win here. Sherlock wouldn't really have much luck even if he had John shooting from a cover like it happened in ASiP, cuz Cas is basically invincible here.
Fight Scenario #2
- Season 6 Cas vs Season 2 Sherlock
Here's where things start to get a bit interesting. Cas was resurrected so he has his angel juice pumping in his veins, but now, even though his people skills are a bit "rusty" he is growing in love with
Dean humanity and he wants the other angels to chill the fuck out. Sherlock from Season 2 is getting his shit together as well. See: The Hound of Baskerville and The Reichenbach Fall. 'Nuff said. Now, both of these idiots have done things to ensure their friends stay safe. Cas lied, Sherlock lied, their friends ended up sad. Where is this going to?
Deduction wise: Cas can still read Sherlock. But now, Sherlock would be able to deduce Castiel's motives or what's going on, I believe. Give him a bit and he has it solved. (oh boy can you imagine what a meeting between Sherlock and Crowley would be like? Have John present to see how Crowley unashamedly flirts with Sherlock and you've got a johnlock confession coming) But it's worth mentioning that Cas would likely be aware of Moriarty and Sherlock's plan too, since he'd pull an uno reverse card if things got petty.
Physical fight: Cas has the upper hand still, but here Sherlock wouldn't really try his luck anyway. I don't think it would be prompted UNLESS for a division of sorts ("Punch me in the face!" "What?" "Punch me in the face, didn't you hear?" "I always hear 'punch me in the face' when you speak")
Conclusion: a tie.
Fight Scenario #3
- Season 9 Cas (human!Cas) and Season 3 Sherlock
So, you see how this is going. Here they even out, slightly. Well.
Sherlock is back, Cas is human, and returning back to their friends is somewhat not as easy as expected. Sherlock has humbled down, and Cas is back after the clusterfuck of s8.
I think it's pretty obvious they think more before acting right now (okay, not Sherlock, not really - see: him leaving the hospital to expose Mary) and as such the deduction game is on the forefront more. Physically, Sherlock wouldn't really want to get into that for more obvious reasons. They're both tired and beaten down and adjusting.
If something happened and they were about to fight, I think it would be even. Sherlock is capable of beating a bitch, and Cas as well. He's been an angel for 99% of his long-ass existence, so there's no way he'd forget how to fight. He is now prone to injuries, though, so Sherlock would have an easier time locating his weak spots, but Cas has that Uno reverse card ready, y'all. At the end of the day, it's 50/50 and depends if they're acquainted or not. They wouldn't hesitate to use whatever means if they posed danger
to the people they love, so that's a factor. If they're friends and it's for a ruse, it's going to be a hilarious clusterfuck.
If they played Cluedo.... I guess Sherlock would get his way of saying the rules are wrong and Cas gives in, not knowing how it should go anyway, so he rolls with it. They both win if teamed up.
Fight Scenario #4
- Season 15 Cas and Season 4 Sherlock
They both cry together man. Take it as a lose or a win-win, up to you.
Now, you could combine this however you like, these are just a few examples. Personally, as long as Cas has grace, he has the upper hand. Although, Sherlock is well able to come up with deception plans to get Cas off his ass, and he'd be able to get into his head eventually, but it'd be a challenge he'd gladly take on.
BUT HERE ARE SOME OTHER SCENARIOS WE CAN CONSIDER
- playing chess: a tie. they are relentless and plan the shit out of it, Sherlock tries to play fair at first but then he wants to see how he can cheat without Cas noticing
- deducing strangers for fun: a tie, but Sherlock has more experience with random encounters, so he'd top Cas by some. Cas isn't dumb, he'd get things right, but some details would escape him, I think, namely reasons for certain things people do or not because he isn't used to living on Earth much. this would be a nice friend bonding experience (one that I'll use at some point soon ;))
- in a rap fight: here I am referencing my Sherlock's obsession with musicals, esp. Hamilton. He'll win every time, or until Cas learned the way of Broadway. Watch Sherlock spill Angelica's and Laffayette's bars like there's no tomorrow
- fight with an angel blade: Cas wins. Angels blades are heavy sons of bitches, and it might take Sherlock a little off-guard in general. Balancing it like a sword and whatnot is a conscious, difficult job I imagine [BY THIS I DO NOT MEAN THEY STAB EACH OTHER just training and fake-fighting]
- having a marathon across London: depends if Cas is human or not. Human? Sherlock outruns you like the brainy bitch he is with the map of the city memeorised. Cas has wings? Blibk blink, bitch, he's there right now.
- having a contest of who has more alcohol tolerance: honestly, I don't know. If Cas was 'on a bender', he drank a whole liquor store and didn't get blood poisoning thanks to angel juice, well. Sherlock stands no chance. If they're both human, they're both wasted. Sherlock knows ash, Cas knows The Real Housewives of Orange County (personal headcanon)
- contest who knows their SO better: a tie. they one-up each other at every given opportunity, tho
Overall I think they're pretty even rivals, at least when Cas is without grace because that put him on even footing with Sherlock. They're both intelligent and can learn from each other, and there are many possibilities as to what they might do, so the jury is out. There's lots more of what I could write but I've been writing this for over an hour, so I'm wrung out :'D so this is just a shallow dive, but it's a very potent question. I'll ponder this in RF too >:D