ElevatedButton

https://stackoverflow.com/questions/49991444/create-a-rounded-button-button-with-border-radius-in-flutter?rq=1
Since, the left sides buttons are now deprecated, use the right sided ones.

Deprecated    -->   Recommended

RaisedButton  -->   ElevatedButton
OutlineButton -->   OutlinedButton
FlatButton    -->   TextButton
ElevatedButton
Using StadiumBorder

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ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(shape: StadiumBorder()),
)
Using RoundedRectangleBorder

enter image description here

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(12), // <-- Radius
    ),
  ),
)
Using CircleBorder

enter image description here

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: CircleBorder(),
    padding: EdgeInsets.all(24),
  ),
)
Using BeveledRectangleBorder

enter image description here

ElevatedButton(
  onPressed: () {},
  child: Text('Button'),
  style: ElevatedButton.styleFrom(
    shape: BeveledRectangleBorder(
      borderRadius: BorderRadius.circular(12)
    ),
  ),
)
OutlinedButton
Using StadiumBorder

enter image description here

OutlinedButton(
  onPressed: () {},
  child: Text('Button'),
  style: OutlinedButton.styleFrom(
    shape: StadiumBorder(),
  ),
)
Using RoundedRectangleBorder

enter image description here

OutlinedButton(
  onPressed: () {},
  child: Text('Button'),
  style: OutlinedButton.styleFrom(
    shape: BeveledRectangleBorder(
      borderRadius: BorderRadius.circular(12),
    ),
  ),
)
Using CircleBorder:

enter image description here

OutlinedButton(
  onPressed: () {},
  child: Text('Button'),
  style: OutlinedButton.styleFrom(
    shape: CircleBorder(),
    padding: EdgeInsets.all(24),
  ),
)
Using BeveledRectangleBorder

enter image description here

OutlinedButton(
  onPressed: () {},
  child: Text('Button'),
  style: OutlinedButton.styleFrom(
    shape: BeveledRectangleBorder(
      borderRadius: BorderRadius.circular(12),
    ),
  ),
)