I am trying to find the Account with most Opportunities in Closed/Won state. Is there any way to achieve this functionality using configuration, without code?
There's no need to use up a custom field on the Opportunity, in order to achieve this.
You can filter the Account roll up summary field to only Count (not Sum) Opportunities where
Won EQUALS True.
See this documentation for more details.
You could create a Formula field on the Opportunity as follows:
IF(TEXT(StageName) == 'Closed Won', 1, 0)
Which would yield 1 if the opp is Closed/Won.
Then, on your Account, create a Rollup Summary that SUMs these. Then, essentially what you'd have is a count of the Closed/Won opps on an Account, which you could then produce a report or list view for.