[Kiwiful lee]

Basketball is amazing cos you can practice alone and also play with randos. Arguably j*cking off is better for a solo sport but basketball's great

[whammabamma]

I don't think anyone said bowling

[Sittesasaa]

Motocross, chess, golf

[maypeach]

Basketball, running, cycling imo

[102345_90]

Hiking,
Swimming

[parker7991 bite]

you can also play soccer by yourself, just do tricks with the ball and learn a better technique

[rock308]

running and hiking. also things like tennis if you want to hit against a wall

[imfrankjordan88]

basketball all day everyday

[LuvIsRage]

basketball best solo sport

[Tommycornrows]

Golf , soccer , rugby

[vbuz1zmsimpxhwtighjrmg]

Basketball's #1 but you could try golf if you don't feel like running around a lot.

[freen254599]

oly lifting

[Mfmm1234]

Golf or basketball

[molotov_corndog]

Cycling is a good one, can potentially be somewhat expensive though

[RocketSurgeon]

Bowling, pool, archery, golf, baseball if you have a ball throwing machine. A lot of sports can be soloed for practice as another user pointed out.

[bigfoodlover123]

I train taekwondo

[Fwjwsh Ejejejeb]

soccer with a wall

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soccer with a wall

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or u can go skiing

[Prophet3771]

for self disciple, your best shot is karate

[tonyazamar28]

depends,, do you mean like: 
completely solo, 100% just you 

or 

solo, but with a team/ coaches or opponent

[ElGrandeBichou]

Dancing off course, great, fun and usefull sports