Pete Sampras
During his 15-year career, he won a record 14 Grand Slam men's singles titles (two Australia Open, seven Wimbledon, five US Open), and had a 203–38 win-loss record over 52 Grand Slam singles tournament appearances.
Roger Federer
He was the World No. 1 ranked player for a record 237 consecutive weeks (from February 2, 2004 to August 17, 2008) and has won 13 Grand Slam singles titles (3 Australia Open, 5 Wimbledon, 5 US Open), currently just one shy of all-time leader Pete Sampras.
Who do YOU think is better?