Pick characters or environments (or weapons, vehicles or props), most jobs want to see you be able to do something from start to finish, so don't expect to just sculpt.
Sculpting I'd definitely go for characters, organic or some environmental stuff. Characters are tough though, a lot harder to make look good than stand alone props.
Yeah like ZacD said, there aren't really many (or any) jobs where the artist just sculpts, you're expected to know how to create a High poly version (sculpt), low poly version, bake and texture. the whole shebang.
if you want to be a dedicated sculptor then you need to show realistic human anatomy, animal anatomy and drapery at the bare minimum and also have environmental sculpting samples as well. having some traditional samples of the above mentioned subjects will also help a lot.
here are few example of artists who can do great being just sculptors only and many of them are just sculptors.
there are many more artists out there who are dedicated sculptors, mainly they are in film and toy industry. there are some occasional ones in games industry but its rare.
it really is super rare to find "just" sculptors in games, and even in your examples, the game artists like david or andrei are not just doing sculpts. They know how to finish assets and they do finish assets.
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Yeah like ZacD said, there aren't really many (or any) jobs where the artist just sculpts, you're expected to know how to create a High poly version (sculpt), low poly version, bake and texture. the whole shebang.
here are few example of artists who can do great being just sculptors only and many of them are just sculptors.
https://www.artstation.com/artist/gionakpil
https://www.artstation.com/artist/loox
https://www.artstation.com/artist/harbottle
https://www.artstation.com/artist/straubulus
https://www.artstation.com/artist/davidgiraud
https://www.artstation.com/artist/assis
https://www.artstation.com/artist/hazardous
https://www.artstation.com/artist/izazelmanska
https://www.artstation.com/artist/samarvijay
https://www.artstation.com/artist/undoz
https://www.artstation.com/artist/sandeepvs
there are many more artists out there who are dedicated sculptors, mainly they are in film and toy industry. there are some occasional ones in games industry but its rare.
You may want to look into texture creation using sculpts as well.
Just some more ideas for ya!