One handed (left handed) bowlines (and a variety of other knots - round turns and two half hitches was quite common, also done left handed… my right was keeping me alive) are an invaluable skill when rigging the sails 15m up in the air on a traditional windmill.
As the saying goes, “One hand for the ship and one hand for yourself”.
It helped that my head miller was a sailor on tall ships, so he had years of experience to pass on.
Just checking. 
For my wife it would be running slalom ski gates. Many people can get through a course, but few really know what they’re doing.
I realize that another skill I could easily peform better than 99 randos is telemark skiing.
Here’s something to consider when choosing a skill. Statistically, 100 random people would consist of…
Okay, I’m not going to be in the top 1% in mental arithmetic then. Some of those countries emphasize mental arithmetic in school a lot more than the US does. I was thinking of with the US population.
I would say archery if I didn’t live in Michigan, where even my Baptist minister used to take time off from church to go bow-hunting.
I don’t have a great deal of skill that I haven’t already channeled into professional work.
I could probably write a novel better than 99 randos, but it’s going to take a few years.
In that vein, how about developmental editing? I have a knack for quickly identifying what an author is trying to do with a work and pinpointing structural issues that get in the way of them doing what they want.** If I were more of a self-starter I could have made a profession out of it.
**Sadly, I lack this skill when it comes to my own work.
Fascinating conjecture.
Anyway…I am blown away by so many of the skills described in this this thread.
I’m having trouble coming up with something for myself. I’ll go with “know (or guess correctly) the etymology of a word.”
Tetherball. If the rules they pick are such that if you foul at all (i.e. stuff like getting the rope caught in your hand), you lose, and it were a single elimination tournament, then I’m not 90% sure I’d beat all 99 of them, such that it wouldn’t be a positive expected value, but tetherball is the only sport in which I’d be well above the average if it were a panel of judges like figure skating or gymnastics. I’ve had multiple people tell me that I look pretty professional playing tetherball. It’s a combination of how good I am (or was, at least), and how I don’t look like I’m hardcore athletic at first glance, so it’s something worth mentioning, and how most people look just plain goofy when playing tetherball.
One time at my previous job we had a work picnic at a local park and we were seated a few hundred feet from a tetherball court. I tried my hardest to get people to play with me by limp-wristedly slapping the ball, but they could tell that I was faking it so no one took me up on the offer when I eventually called out to people to join me.
Hehe, do you mean drive a car? Fast? If it’s a drag race in a straight line, I’m almost certainly better than 99 randos. I can cut very near a .400 light most of the time. I don’t have a ton of experience with high horsepower (400hp+ 2WD) vehicles, but I can handle launching cars up to that level without any computer assistance, at least.
If it’s a speed contest with curves, I’m decidedly average. If you have a contest with 100 drivers who are interested in the sport, I’m statistically likely to end up finishing 50th. So, I’ll still probably do well against 99 randos, but it’s less sure than it would be in a straight line competition.
If it’s a contest to get a passenger safely and calmly from point A to point B? Ehh, I could do well, but it would be a crapshoot depending on if I could imagine the ghost of my dad in the car. He apparently told my mom he thought I was a better general driver than he was. But when he and my mom were in the car, I was trying to be the best chauffeur I could imagine. I didn’t want him to think I was a maniac, after all. Conversely, I’ve had a co-worker tell me “I always want to ride with you because I never think I’m going to die, but I know it will be exciting!” So, yeah, the experience might depend on mine and what I think the passenger’s expectations are of the trip.
Oh trust me if you are a pro instructor you’re gonna be better than I am. But 99 randos, well I still fancy my chances against at least 90 of them I guess.
TIL: Bob Holness was born in South Africa. And moved to the UK when he was 7. Still British but not British born, which I didn’t know.
Orienteering but only if I’m allowed to spend a few months getting running fit.
I grew up in a family that lived and breathed orienteering (it was our sport and our social life). I had no idea how much better I was at the skills required for orienteering than most people because everyone around me was equally steeped in the sport.
Till one year I went on a school camp which had an orienteering activity. The instructor gave us a lesson then sent us on a course. I wasn’t sure how well I would do because I knew that when it came to long distance running (which is an element of orienteering) I was barely mediocre compared to many others.
I can’t remember the margin by which I beat everyone else but it was massive.
The catch is I haven’t done any running for decades. So I’d probably keel over and have a heart attack if I tried to orienteer at speed against 99 other people tomorrow.
But if I was running-fit I think my navigational ability would put me ahead of 99 random people, easily.
Word! I never skied less than when I skied more. 
raising one eyebrow, then alternating raising first the left and then the right. The unraised eyebrow does not move.
Wherever you’re getting that from is out of date, it should be more Indians than Chinese.
I would bet that you could WALK your courses in an orienteering contest against 99 randos. Assuming, of course, that all they have is the instructions and a compass. I learned basic orienteering in Boy Scouts, which came in handy when I took a crash course in land surveying, but it’d be completely baffling to the vast majority of people.
I agree that I could probably come (at least) top ten in my current state of fitness against 99 randoms, for the reasons you state.
But as matters stand, any person who knew the basics and could run a few k would beat me. I suspect there is a reasonable chance that a group of 99 randoms would contain one such person.
So, until @Elmer_J.Fudd and @colinfred pointed out the preponderance of Indians, my top pick in a list of “skills I have no more than a basic competency in, but which are quite rare nowadays” was: leg-spin bowling. But up against c.20 people from a cricket-mad nation, I am rethinking rapidly. That leaves me with one of:
Translating Latin (have the exam certificates to prove it, and the passage of time to explain why I’m rusty)
Playing bridge (bit of a gamble here, am also rusty but it’s not a common game any more - hence the rust)
but I think the best bet is:
Solving cryptic crosswords. Has the additional advantage that as far as I know they are mainly an Anglophone pastime, which gives me a massive unfair advantage but look, I don’t make the rules.
heh. Yeah. I’ve been driving for 50 years and have never been in an accident, regardless of fault. No one has hit me, I haven’t hit anyone.
I’d say I could outswim 99 other random people. I swam in high school, and have been swimming as a Masters since 2001. Looking at qualifying times for World Masters I think I’m right on the line, so I expect I could out swim most people. If I go open water for 5k, 10k, 15k, or 20k, I don’t think most people could even finish. Though I am not in shape for anything above a 10k.
I’m pretty sure I could beat 99 random people on an original Mortal Kombat 1 arcade machine. I have one in my basement and I’ve been playing since it came out.