Gregory 'The General' Gaultier
Whereas most pros (and recreational players) will have their rackets strung between 22lbs - 30lbs, the General has his rackets strung at around 16lbs with Tecnifibre 305 1.1mm or 1.2mm. He has been known to go as low as 13lbs. This gives him easily accessible power. The fact that he can control the ball at this tension is testament to his skill level. Gregory Gaultier, the exception to the rule.
Mohamed 'The Beast' Elshorbagy
Mohamed is known for his relentless hitting and 'beast mode' squash. He has the capacity to batter his opponents off court. You will normally find Mohamed using the Tecnifibre Dynamix VP 1.15 or 305 in the low to mid 20's - giving him a good level of power to suit his game as it's evolved.
Joelle King is one of the top players on the women's pro tour and uses Ashaway PowerNick 18 (red). Joelle typically strings her rackets at up to 28lbs, which is on the high end, however she uses a larger racket head. The strings feel very different from most and are a real connoisseur's string. I wouldn't recommend for inexperienced players, but if you like them, you'll probably never switch again.
Roger 'G.O.A.T' Federer
Roger Federer, most likely the greatest Tennis player to walk the earth, is known to use Wilson Natural Gut for his mains strings and Luxilon Alu Power Rough for his crosses; to allow him comfort, power and spin. Most tennis players use different tensions depending on the type of court and climate, so it's difficult to tie Roger down to a specific tension, though 45lbs - 55lbs is a fairly safe bet , with the Natural Gut being strung higher than the Alu Power Rough.