'She talks least but does most.'
'She talks less but does most.'
(You are solely trying to describe this particular girl's character.)
As for this sentence, 'She talks the least but does the most.'
You are actually trying to pin point a particular girl's character in a group.
For example, 'Amongst all my siblings, she talks least but does the most.'
Placing of punctuation mark 'comma' also changes the meaning of an entire sentence.
Hope this helps..