Translation:Who is your best friend?
Duolingo is my best friend and my only friend...
I think "Who is your closest friend?" also works.
same thought - should also be accepted
What is the difference between 제일 친한 and 가장 친한 ?
Can't say for certain, but I know 제일 means something like 'number one' while 가장 means 'most'. I also know they're mostly interchangable and kind of mean the same thing.