If you could pick just one sport to be good at, which one would it be? For me it would be skiing: I’ve only skied a few times and I’m rotten at it, but seeing the really good skiers who make it look effortless, and getting glimpses of the rush of speed and exhilaration when you get one smooth straight run - yeah, it’s addictive; I can see why some people spend so much time and money perfecting their skills. I’d love to be a good skier.
Soccer. I was a pretty good goalkeeper once upon a time, I would have loved to have had the chance to do it professionally. To have my skills enhanced to the global professional level would be amazing.
If I had a choice when I was a teenager, it would’ve been ice hockey. I dreamed of being the best goaltender. (I’m only 5’8", so what else?)
It’d be great to play a game and I get snowed by some Ranger/Devil/Penguin asshole. Then I turn and say to Mr. Berube or Mr. Lindros, “Hey, that guy just snowed me.” Next face off, the guy from Rangers/Devils/Penguins ribs are broken. Cheers erupt. It’s like being the Don of a very small mafia on skates. 
Depends on how good. If I were the right age and good enough to be a top professional, I’d take baseball. Nice long carer available, probably won’t suffer a concussion, and my favorite sport. If I can only be good like an “ordinary” person, I guess I’d pick tennis. At my age now, I guess I’d take running so I’d be in good shape.
Climbing and/or parkour. Mainly for urbex purposes, but being able to spider up cliff faces or wall-jump clear over somebody’s head sounds like it could come in handy in a pinch.
I’m 35, so at my age probably something like golf that I could play for 30+ years.
For a short career I’d pick ice hockey. The skill set is amazing. There is nothing that a hockey player does that is natural. Skating and handling a puck with a stick are entirely learned skills.
For a long career, probably golf. To be really good at it can open a lot of doors and you can play it pretty much all over the world. To be confident enough to hit a good shot under almost any circumstance would be really rewarding.
Just dreaming, but to be able to turn around a 90 mph fastball would be really cool.
Deeply honest answer…world class swimmer or Iditarod competitor.
tsfr
all good answers imho…
mush forward!
Telemark skiing.
Golf.
You can make a very, VERY good living and still be relatively unknown unless you’re in the top 10, so you can go out to eat or the movies without getting mugged. You get to walk around beautiful courses and travel the world, plus there’s a senior tour where you can keep earning big bucks after you lose your edge.
Tennis. While I don’t think I would ever have been interested in going pro even if I was good enough (I don’t have that kind of competitiveness), I still think I could have been a decent player if the teacher hadn’t been a twat who insisted in making me play right handed (where I couldn’t hit the ball at all) rather than lefty (where I did both hit it and put it where I wanted it, damnit).
Good is such a relative term.
If good merely means in the top 10% then I would choose gymnastics because the skills carry over well to a lot of real world challenges.
But, parkour is a very close second for the same reasons.
Probably golf. I’m over 50, so it’s the only thing my tired old carcass could handle. Benefits also include big bucks if you’re any good and never having to play in bad weather.
If I were young, I’d choose swimming. I have a feeling you young folks are going to need to be able to swim.
Mixed martial arts.
I’ve lost more than my share of real-life fights. It’d be nice to win the next time.
Soccer.
I’d settle for a good second-division player level.
Major-league level? Baseball. Less damage, pretty long career, solid money. Basketball is probably better in those regards, but I don’t really like that sport very much.
If it’s just “good” in the amateur sense then probably golf or tennis. Both can be enjoyed later in life and can be “toned done” to play with/against friends and family.
I’d probably sell my soul, worn out as it is, for ten years or so as a good all-rounder in pro baseball. My ten days with the Giants weren’t enough.
At the professional level? Soccer. Debilitating injuries are unlikely and you can work in a lot more different places than most sports.
Golf’s a good back up. I like the anonymity part that was mentioned.
For the reasons mentioned about, baseball if it’s on a professional level, golf if it’s on a semi-professional level.