It really depends on your personal preference on how you want it to sound. This song has been done millions of times, and can be interpreted in many ways, there is no right wrong or best, just preference. Any slow ballad style will work once you set the tempo, eg swing ballad styles, Midnight Swing, big band ballads. I even like the 6/8 soul styles for this kind of slow ballad.
The best thing is for you to learn how to find and try styles for yourself rather than ask for recommendations. This will set you up better for the future.
Try doing this : tap in the tempo you think you would like to play this at. Select a ballad style from the swing or big band set on the keyboard. Play the first couple of chords of the tune while you hum the tune in your head. Then do the same with a different style, but at the tempo you have selected. Try several until you find a few possibilities, and pick the one you like best. Sometimes the unexpected styles work great when slowed to the the desired tempo. This is the way we all find styles.
BTW, this is a very well known standard, performed by many people but is probably best known for being Liberace's theme song. He ended his shows singing it.
Mike
