For me just about every Sinatra recording is just brilliant especially his recordings for Capitol Records and Columbia. There is a lot of magic on his Reprise records as well.
His interpretation of popular song was like listening to the greatest actor in the world doing Shakespeare.He made you appreciate poetry when you didn't even know you liked poetry. He could set a mood like no other. He chose the some of the best popular songs written by some of the best song writers of the 20th century. He had some of the best arrangers and best musicians around.
Obviously as he reached his fifties his voice became less mellow but I went to see him in a concert at the Royal Albert Hall in London in the ninteen eighties, there wasn't an empty seat in the place and boy did he make that place come alive.
I've heard it said that when he first started out and singing with the Tommy Dorsey Orchestra everyone on the dance floor just stopped dancing and watched him, mesmerised by his voice.
I must mention another great singer who was under estimated and unfortunatley died last year his name was Al Martino , he was not as prolific as Sinatra but some of his recordings such as, " To each his own, " " Fascination, " An Affair to Remember, " " I have but one heart, " " Blue Spanish Eyes, " Nessun Dorma ," etc. etc. are fabulous.
Just a few thoughts.