1.18 or 1.24 or 1.32mm, for best performance.
VS Touch available in 16, 1.30mm or 15L, 1.35mm weatherproof.
VS Team natural gut 17, 1.25mm best string for performance and comfort, used by Roger Federer to great effect made in France.
15L, 1.35mm natural gut, weatherproof, thicker gauge last longer.
For better performance and durability brush the string bed from time to time with gut oil or baby oil.
Please don't use natural gut in wet weather conditions, it will be ruined in no time.
17 or 16, multifilament core for great performance