FYI: सबसे literally means "from all"
I think the English statement might be better as "That boy is the most handsome".
I think सुंदर pretty directly translates as beautiful. There are other words that maybe imply handsome a little more. Perhaps रूपवार (shapely), although I'd really need a mother tongue speaker's opinion here.
The English sentence should have had "handsome"
I'm from india and sundar is both beautiful and handsome